China plans to implement the navigation services of BeiDou Satellite Navigation System on a global scale in 2020. To ensure the acquisition in BeiDou-challenged environments, this paper proposed a two-step acquisition method to eliminate influence of bit sign transitions. To reduce the Doppler estimation time, the INS information is integrated to detect the receiver's speed. As satellite ephemeris information is downloaded by the assisted receiver, with the aid of INS, the relative speed of receiver and satellite can be obtained, and the Doppler shift searching range can be reduced within ±300 Hz from ±5 KHz.