TDOA location is a high precision location technique. The satellite size is limited to the length of the base line, so that the location accuracy of the time difference on satellite relates mainly to the accuracy of time difference measurement. The Time Difference Measurement based on signal phase and the factors relative to the time accuracy are introduced. The principle of Time Difference Measurement based on signal phase and the block diagram are provided. The factors relative to time accuracy are detailed. The influence of the effective bits number of AD on time accuracy measurement is simulated. The results showed that the method of increasing the effective bits number of AD can greatly improve the accuracy and achieve time accuracy in ps