In the future, integrated GPS-Galileo Navigation Satellite System will expand its applications in many fields. For space robots, it will provide high accuracy and real-time navigation service. As it is well-known, the integrated system will provide twice and more satellites than individual system. Therefore, selecting appropriate satellites from the combined constellation is a very important aspect. Using a reasonable and fast algorithm to select satellites would have a favor to precise and real-time navigation. Integrated GPS-Galileo is a mixed-constellation navigation system consisting of satellites of two different types. Taking the different satellites have different range errors into account, we choose the Weighted Geometric Dilution of Precision (WGDOP) instead of Geometric Dilution of Precision (GDOP) as the optimal satellite selection criteria and propose the new satellite selection criteria-WGDOP minimum algorithm for the combined GPS-Galileo navigation receiver. The algorithm makes full use of available information to select the optimal satellites for navigation solution. And the selection algorithm is simulated and validated by a software simulation. The results show that the WGDOP minimum criteria can more accurately reflect the performance of combined constellation and evaluate the positioning accuracy. And also it can improve the navigation accuracy and reduce the measurement error.