The invention discloses a light controlling device for measuring penetrant level of a soil sample, comprising a transparent glass tube, a T-shaped guide pipe, an opening container, a light source component, a sensor module, a signal processing machine and a computer, wherein the T-shaped guide pipe is vertically inserted in the opening container; the light source component comprises a modulation signal generator and a linear light source; the sensor module comprises a light detector, a preamplifier, a radiodetector and a digital signal processor; the linear light source is matched with the light detector; the linear light source and the light detector, facing to each other, are arranged at two sides of the transparent glass tube in transverse direction; the signal processing machine comprises a data input terminal and a data output terminal; the data input terminal is connected with the digital signal processor; and the data output terminal is connected with the computer. The light controlling device overcomes the defects that the reading is inaccurate and the measurement can not be done in real time when liquid level data are observed by naked eyes, and is an electronic measurement device that can fully and automatically measure the penetrant level of the soil sample.