Influence of coupling agents on microstructure and mechanical properties of short glass fiber reinforced PA66 composites(SGF/PA66) were investigated by using twin screws extruder and injection machine in this paper. When coupling agents (A1100 or A1100+A+B) were added, short glass fiber was distributed in the PA66 matrix more homogeneously; at the same time, the microstructure and properties of GF/PA66 were improved too. Modified effect of multiple coupling agent (A1100+A+B) is better than that of only A1100 and the desired content of A1100 is about 1.5~2.0wt%. When glass fiber content was less than the critical value (35~40%), mechanical properties of PA composites increase with fiber content increasing, but it begin to decrease when content was excess that value. At last, Failure mechanism of GF/PA66 (treated by A1100 or A1100+A+B) was obtained: adhesion of interface between glass fiber and PA66 matrix, friction after the adhesion, glass fiber pullouted and matrix failure.