To improve the time synchronization precision of a three-line stereo mapping camera, a time synchronization error model for the three-line stereo mapping camera was set up based on the analysis of working principle of the time system in the camera. Then, several methods such as reduction of row synchronization counter polling period were adopted to optimize the time system. As the time synchronization errors may change with the working time sequence of the camera controller, a real-time time synchronization precision measuring method for the three-line stereo mapping camera was put forward to measure and record the dynamic changes of time synchronization errors in real-time during photographing process globally. With the method, the difficult problem that custom instruments could not be used for long time measurement of time synchronization errors because of their finite memory depths was solved. This method was used to measure time synchronization precision of the time system for the optimized three-line stereo mapping camera. Experimental results indicate that time synchronization errors of the camera are no more than 74.8 μs during photographing process and the probability of absolute value of time synchronization errors no more than 11.2 μs is more than 0.95, which meets the demand of qualification.