Underwater lidar technology has become an important tool for ocean exploration and underwater operations. So far, all of the reported underwater lidars are based on the pulsed Time-Of-Flight(TOF) method for ranging, which is characterized by high pulse peak power and relatively long detection distance. However, TOF lidar is large in size, high in power consumption, and small in dynamic range, and there will be interference between different TOF lidars. Hence they are not suitable for compact platforms such as Autonomous Underwater Vehicles(AUV) swarms. In addition, back scattering presents a great challenge to underwater pulsed lidar, which limits the sensitivity and dynamic range of the detector. Strong back scattering of suspended solids in water can completely submerge the real signal, leading to false target detection. Underwater Frequency Modulated Continuous Wave (FMCW) lidar has a greater dynamic range than TOF radar and can output a continuous spectral signal proportional to distance. Because the signal of the coherent detection is amplified by the local oscillator, FMCW lidar requires low laser output power and can be realized with low power diode laser, which can significantly reduce the size, weight and power of the lidar system. So far most reported studies on FMCW lidar are concentrated on near-infared wavelength such as 1 μm, 1.55 μm. However, there have been no reports of underwater FMCW lidar using blue or green lasers so far.The implementation of FMCW lidar is faced with two challenges, one is to obtain a single-mode tunable narrow linewidth laser, the other is to eliminate the nonlinear effect of laser frequency modulation. Because the light absorption loss of water will lead to the signal attenuation and shorten the detection distance, it is necessary to select the blue-green laser located in the seawater “transmission window” with a small attenuation coefficient. A commercial 405 nm Blue FP laser diode and a 30% reflector are used to construct an External Cavity Diode Laser(ECDL). The blue ECDL exhibits a coherence much longer than 10 meters in fiber, and a continuous frequency sweep range of 1.5 GHz by direct modulation of the injection current. Modulation nonlinearity of the FMCW laser can broaden the FFT spectrum of the measured signal and reduce the ranging accuracy. To overcome this problem, an FMCW ranging method based on equal optical frequency resampling is adopted. The system consists of both a fiber interferometer and a free space interferometer. The optical fiber interferometer uses 10 m delay optical fiber to monitor the frequency change with time, and the free space interferometer is used to obtain the beat frequency signal containing the distance information of the target to be measured. Theoretical analysis and experimental verification were carried out. The underwater ranging was conducted by adding Mg(OH)₂ powder in the water tank to adjust the attenuation coefficient to simulate the seawater environment. An FMCW laser ranging system with dual interferometers is built, and the extremum point of the beat signal of the fiber optic interferometer is extracted to sample the beat signal of the free space Interferometer. It is verified that the beat signal spectrum broadening problem due to the laser′s frequency modulation nonlinearity is well corrected. Using the resampling method, the distance is measured at eight equally spaced positions, and a reflector is placed at these positions in turn as the detection target. The results show that the measurement range of the target is linear with the actual range, and a detection range of 5 meters is realized with only 9.5 mW laser output power, and the maximum error is 0.051 m. In order to obtain the measurement error of the FMCW ranging system, ten groups of data are measured at the same position. Through calculation, the average measurement error of the ranging system is 0.027 m. Our work demonstrates that a low power commercial blue laser diode can be used to construct a single frequency ECDL laser, and it can be directly frequency modulated to perform accurate underwater FMCW laser ranging. This lidar architecture characterizes by small size, low power and weight, which promises an effective laser ranging sensor for compact AUV swarm.