Space charge is an important factor leading to insulation aging under square wave electric field. However

it is difficult to directly detect the space charge in insulation under square wave electric field

due to the quickly polarity reverse at the level of ns to μs. In order to implement direct detection

the core parameters of space charge detection equipment in an examination system by a pulsed electro-acoustic method（PEA） were analyzed to determine their influence on space charge detection under the square wave stress. The direct dynamic detection of the space charge under the square wave stress w

as realized by the automatic equal phase shift principle（AEPS）. The formation mechanism of space charge under the square wave stress was discussed preliminarily by detecting the space charge with polyimide film under the square wave stress of 10 kV/mm. The results show the protection resistor R

1

the coupling capacitor C

1

and the sample capacitor C

2

constitute a low-pass filter circuit for the PEA system. A large time constant τ may cause seriously distortion of square waveform applied on tested sample. It is indicated that the space charge detection during instantaneous polarity reverse of square wave stress can be realized for the system with small τ by reducing the values of R

1

and C

1

.The direct space charge detection with high phase resolution about 1.79° is realized under 10 kV/mm for polyimide film by AEPS principle. According to the space charge behavior under the no-overshoot 10 kV/mm square wave with the different rise time

i.e. 100 μs

500 μs and 1 000 μs

the internal space charge mainly comes from the homocharge injected from the low electrode with aluminum material. The space charge can be quickly extracted during the polarity reverse process

and no significant space charge can accumulate inside the specimen.