Traditional laser range finder based on two-step frequency mixing has a poor phase measuring accuracy and it is difficult to eliminate systematic errors. A novel quadrature modulation method to achieve amplitude modulation of laser intensity is proposed. This method improves the accuracy of phase measuring and greatly simplifies the hardware design of laser range finder based on two-step frequency mixing. Design features of the prototype have been expounded in detail. A highly integrated quadrature modulator chip is used in prototype design, making the whole system compact and with no redundant components. The measured accuracy approaches ±1.52 mm when the modulation frequency is 62.5 MHz and the dynamic range can be improved by multimodulated frequency method as revealed by a detailed analysis.