We propose a laser rangefinder system based on the quadrature modem to achieve amplitude modulation, the method improves the accuracy of the phase measuring and simplifies the hardware design compared to the system with the secondary mixing methods, and to solve the range ambiguity caused by the measuring process, the ranging ambiguity resolving algorithm based on the over-determined equation is proposed, which avoids the searching of the optimal solution, finally the two K60 laser rangefinders and three proposed rangefinders were experimented on the national standard baseline with the precision of 0.18 mm. the measuring time of the proposed system is less than 1.8 s, the average measuring error of those three prototypes is less than 2 mm, and the standard deviation is less than 1 mm within the measuring range 0~60 m. The experimental results show that the proposed design system of the rangefinder has higher measurement accuracy and speed in comparison with the traditional ones, which indicates the high reliability of the proposed design system.