Weak RF signal detection with high resolution and no blind zone based on directly modulated multi-mode optoelectronic oscillation has been proposed. The high-sensitivity optical modulators and optical filters are avoided because multi-mode oscillation is obtained based on directly modulating the semiconductor laser at the relaxation oscillation frequency. For the directly modulated optoelectronic oscillator, the detection characteristics such as gain for the RF signal, resolution, noise floor, and sensitivity are firstly analyzed. The experimental results are consistent with the simulated results. For the RF signal of unknown frequency, it can be detected out and amplified by tuning the bias current and delay time of the loop. There is no blind zone within 1–4.5 GHz. The system provides a maximum gain of 17.88 dB for the low-power RF signal. The sensitivity of the system can reach as high as $-95\mathrm{dBm}$. The properties such as gain dynamic range and power stability are also investigated. The system has potential for weak RF signal detection, especially for the RF signal with unknown frequency.