The direct algorithm of interferometric synthetic aperture radar measuring is to directly introduce the absolute phase difference into range equation and Doppler equation to resolve the ground coordinates. By using the ideal image coordinate, absolute phase difference and orbital position of InSAR which include reasonable errors as the initial data, we can get the ground coordinates which include the errors and realize the analysis of geometric accuracy of the ground coordinates. In order to introduce the errors of the baseline length and attitude into orbital position, we propose the corresponding mathematical model. The computation results show that the accuracy of direct algorithm is almost the same as that of the classical algorithm.