An ultra-weak fiber Bragg grating (UW-FBG) strain rosette sensor based on SUS304 metal substrate is proposed. The working mechanism of the sensor is analyzed by theory and ANSYS simulation, and an equilateral triangular SUS304 strain rosette substrate is designed. Three UW-FBG sensing units with central wavelengths of 1536, 1542 nm, and 1548 nm are fixed to the substrate by 353ND epoxy resin glue to fabricate strain rosette sensor, and the sensors are calibrated for their temperature,tensile, and compressive properties, the proposed sensor is applied to monitor vector strain of the jacket model. Results show that the strain rosette sensor has a average temperature sensitivity of 25.4 pm/℃, with linearity of ~0.9990. The average compressive principal strain error of proposed sensor is 1.74%, with principal strain angular error of 0.51%, and a tensile principal strain error of proposed sensor is 3.22%, with principal strain angular error of 0.59%. It has a good reproducibility of experiments, which indicate that the proposed sensor can provide an excellent solution for vector strain monitoring.