The 4 mask process of TFT substrate can improve production capacity, but it also brings the problemsin product quality. For example, the 4 mask process can generate white dot defects. The causes of white dots is explored through DC experiment. The mechanism of white dots is studied by illumination experiment and high temperature experiment, and the white dots defects are improved by process adjustment. The mechanism of white dots defects is that the conductivity of active layer is different due to SW effect, resulting in different feedthrough voltage (ΔVp), and the difference of optimal Vcom leads to white dots defects. With the gate and drain electrode signal coupling capacitance Cgs becaming smaller (i.e. the smaller active tail width), the white dots gradually decrease until they disappear. With the pixel storage capacitor Cst becoming larger, the insulation layer thickness (PVX) is reduced from 600 nm to 400 nm, and the degree of white dots defects can be reduced by 1 level. After the process adjustment, the width of active tail is reduced to 1.2 μm, which can solve the white dots problem caused by 4 mask process. This work provides effective solutions and references for the improvement of product quality and benefits as well as subsequent product development.