A turnable diode laser trace gas measurement device and method which use a high diffuse reflection square chamber to increase optical paths belong to the technical field of trace gas concentration measurement. The invention solves the problem that methods which increase gas absorption optical paths in a limited space are complex in structure and high in cost. The device comprises a diode laser and a high diffuse reflection square chamber. The device further comprises a photomultiplier tube, a sawtooth wave signal generator, a sine wave signal generator, a mixer, a temperature controller, a current controller, a data collection card, a computer, a high voltage power supply, a wedge incident quartz lens and a wedge exit quartz lens. The method demodulates an electrical signal output by the photomultiplier tube into a second harmonic signal in the computer to obtain an optical parameter OP of the concentration of a to-be-measured gas, and then compares the OP to a standard curve of a to-be-measured gas optical parameter changing with the concentration of a gas in a high diffuse reflection square chamber, to obtain the concentration of the to-be-measured gas and to achieve measurement of the content of the to-be-measured gas. The device and the method are used for trace gas measurement.