The experiment result of series photoconductive switches triggered by ns laser pulse is reported. The series switches is composed by two semi-insulating GaAs photoconductive switches with 3mm electrode gap and the electrodes at two sides. Both electrodes touch each other. When it is triggered by ns laser pulse about 2mJ at 532nm, with the biased voltage of 1200V, linear mode of the series switches is observed. Then, laser energy and the biased voltage add to 5mJ and 4000V, the series switches still works in linear mode. When it is triggered by ns laser pulse about 6mJ at 1064nm, with the biased voltage of 3500V, the series switches works in linear mode. When laser energy and the biased voltage are 10mJ and 4000V, the series switches gives a double wave crest waveform. However, with the biased voltage of 6700V and laser energy of 15mJ, series switches gives trend of nonlinear mode. When laser energy rises to 20mJ, nonlinear mode is clearer. Even at 5000V and 30mJ, trend of nonlinear mode is observed. The results indicate that double wave crest appears while series switches works from linear mode to nonlinear mode. Capacitance effect caused by electrodes of switches touch is discussed. And explanation is given that double wave crest is attributed to surge caused by capacitance. When biased voltage and laser energy are enough, high gain of carries is very obvious. Effect of capacitance decreases and double wave crest disappears. Then the series switches is going to nonlinear mode. This explanation matches experiment result.