Droplet formation in a microfluidic T-junction is simulated by using computational fluid dynamics (CFD) method. The accuracy of CFD study is verified by comparing the simulation result with the experimental results in literatures. It is found that the formation of droplets in the T-junction can be divided into 3 steps: droplet emergence and growing up, separation with the dispersed phase, and detachment from the channel wall. The study on the wetting property of the channel wall shows that it has a remarkable influence on the droplet formation. Only when the contact angle of the dispersed phase on the wall is smaller than 90°, the droplets can be formed. Moreover, the wetting property of the channel significantly affects the detachment time of the droplets from the wall. The effect of the capillary number, Ca, on droplet formation is studied by varying the viscosity, and velocity of the continuous phase or the interfacial tension, respectively. In the range that the droplets can be formed, the droplet size decreases with the increase of Ca，and when Ca is bigger than 0.067, droplet diameter shows an obvious linear relationship with 1/Ca.