Smoothness of the spatial curve of trajectory planning was researched. It was put forward to ensure the C2 continuous sinusoidal acceleration curve to control the speed of circular arc track, and using quaternions spherical linear interpolation planning a smooth circular trajectory of robot endeffector. Finally the simulation and experiment on six joints handling robot were conducted. The results show that using the sinusoidal acceleration curve of speed control, not only the C2 continuous is guaranteed, but also the amount of calculation is relatively small, which can well meet the realtime requirement of robot control.