In order to address the problem of the severe nonlinearity in the selfalignment loop of the platform when the large angle misalignment exists on the stationary base, a new large angle selfalignment algorithm of HRG is designed based on the unscented particle filter (UPF), and the numerical simulation and rotating platform simulation have been carried out. The results show that the superiority of this algorithm which made a contribution to the application of the HRG to the initial platform will be demonstrated through theoretical analysis, digital simulation and experimental results.