The present invention provides a device for testing micro electric charges and a method for testing micro electric charges by dint of the device. Improvement is done on the basis of the traditional field effect transistor. Not a grid electrode is arranged between a source electrode (5) and a drain electrode (6), and a dielectric layer (2) is mixed with electric charges between 75 percent Qid to 125 percent Qid, a probe (4) is fixed on the dielectric layer (2); wherein, Qid can be seen in the figure. The method for testing micro electric charges of the present invention comprises the testing steps: A voltage is added between a grid electrode (7) and an underlay (1) by using the device of the present invention; a fixed bias voltage is added between the source electrode (5) and the drain electrode (6), and electric current between the source electrode (5) and the drain electrode (6) is measured; reacting substance and the probe (4) are added to release the measured charged; the voltage bias between the grid electrode (7) and the underlay (1) is adjusted to guarantee the invariability of electric current between the source electrode (5) and the drain electrode (6); variable quantity of the bias voltage of the gird electrode (7) is measured; micro electric charges on the surface of the dielectric layer (2) is worked out.