Abstract: In view of the observation data of BDS-3 at 5 frequencies and the theory of un-differenced and un-combined (UDUC) precise orbit determination, this paper introduces the UDUC model and parameter estimation method, proposes the strategy of station selection using the K-means algorithm, and analyzes the advantages of UDUC method. Through two kinds of station selection schemes, manual experience selection and K-means, 3 frequency selection methods of BDS-3 5-frequency, B1C+B2a, B1I+B3I are used respectively, and 30 IGS observation stations are used to carry out precise orbit determination for BDS-3 MEO and IGSO satellites. The experimental results show that when the stations which can receive B1C+B2a frequency observation data are insufficient, the UDUC method can increase the number of observation data and optimize the station layout by using the 5-frequency observation data so that the orbit determination accuracy can be improved. Compared with B1C+B2a, the monthly average RMS of 5-frequency in A, C and R directions increase by 0.003 m, 0.004 m, 0.003 m respectively, and that of 3D RMS increases by about 0.007 m. The stations selected through the K-means algorithm are more reasonably distributed and have higher orbit determination accuracy than manual experience selection scheme. With the 3 frequency selection methods, the monthly average RMS of MEO satellites in A, C and R directions are improved by 0.009 m, 0.017 m, 0.009 m respectively.