A detection and counting unit with optical fiber was designed for realizing the detection and counting of micro-droplets on the microfluidic chip. TracePro was used to simulate and provide the basis for detection signal processing. The system can realize the counting function according to the change of the optical intensity, which is caused by droplet passing. Simulation were carried out to analyze the influence of beam collimation, droplet size, droplet refractive index and distance from receiving fiber to chip on optical intensity. The results obtained from the simulation show that the poorer the collimation beam, the larger the droplet radius, the smaller the refractive index, and the closer the distance between receiving fiber and chips, which can cause more apparent variation in optical intensity. What’s more, the droplet size determines whether there are double valleys in the optical intensity curve. When the radius of droplet is less than 13 μm, there is one valley in the optical intensity curve. And when the radius is larger than 17 μm, there are two valleys.In addition, processing method of detecting signal was proposed based on the simulation results, showing that this optical detection unit can realize the detection and counting function of the micro-droplets on the microfluidic chip.