With the completion of the BDS-3,China has become the third country that has global navigation satellite system.Furthermore,BDS-3 can broadcast observation signals up to five frequencies,which has great significance to achieve rapid fixing of ambiguity and improve positioning accuracy.The Ionosphere Reduced (IR) combination has minimal ionospheric delay and integer characteristics,therefore,a solution model based on IR combination is established for medium and long baseline utilizing the quad-frequency observations of BDS-3 in this paper.In the meanwhile,the tropospheric delay is estimated with tropospheric mapping function.The experimental results show that compared with traditional dual-frequency Ionosphere-Free (IF) model,the ambiguity fixing speed of the IR model is improved by more than 10％,and the positioning accuracy in north (N),east (E) and up (U) directions are improved by 7.7％,7.9％ and 8.2％,respectively.