In view of the fact that 70% of the faults of hydraulic equipment caused by oil contamination， solid particles are the main contamination. Most of the commonly used oil contamination detection technology and equipment have the status of complex structure， expensive price and inconvenient online detection. Herein， a reflective optical fiber oil contamination detection sensor is designed. According to the attenuation of light intensity caused by the blocking of light by contamination particles， combined with the propagation characteristics of light in liquid， the working principle of oil contamination degree reflective optical fiber detection is proposed. The mathematical model of the sensor is established according to the approximate Gaussian distribution of the optical field at the end of the optical fiber. The influence of different parameters on the output characteristics of the sensor is analyzed. The structure of the sensor probe is determined. The reflective optical fiber sensor for oil contamination detection is designed. Based on the sensor test platform， the standard oil is configured according to different contamination levels to verify the characteristics of the sensor. Experimental results show that the sensor can detect the hydraulic contaminated oil with different contamination level standards.