In the standard water device, using an AC drive controlled by computer the rotating speed and flux of pump can be adjusted smoothly. However, some ultra instable factors in the system may be introduced in addition. By investigating a lot of experiment data systematically, the ultimate results of comparison between the operations handled by personnel and by computer are presented. Some analyses and discussions about the reasons and amendable measures with regard to the instability of system aroused by AC drive is involved in the paper.