Abstract: One of key issues in the aspect of UPS-based attitude determination that must be solved is to estimate the unknown integer ambiguities. Single-epoch attitude ambiguity resolution can be achieved by using MC-Lambda method，which fully exploited the known geometry constraints of the multi-an-tennae configuration. This method does not need to consider cycle slip problem. However，the float solution precision of integer ambiguity is low based on UPS pseudo-range and carrier phase observations，which leads to large ambiguity search space and low search efficiency. For this reason，choosing the direction cosine matrix to describe the attitude and UPS/Uyro attitude determination model was established. The float solution of integer ambiguity was calculated by matrix kalman filter algorithm，and then the fixed solution of integer ambiguity was obtained by MC-Lambda method. Simulation experimental results showed that the accuracy of float solution by Kalman filter algorithm with constraints of direction cosine matrix are improved，so the computational efficiency and the fixing success rate of the fixed integer ambiguity are all improved，particularly when UPS observations is in the bad observation condition.