Abstract: The integrity of satellite navigation systems is attracting increasing attention in the safety-critical applications, such as civil aviation. Advanced receiver autonomous integrity monitoring (ARAIM) is a low-cost, highly autonomous integrity monitoring method. This paper proposes a method of calculating the protection level based on the optimal allocation of risks, in order to solve the problem that the integrity risk and continuity risk are equally allocated, which results in a conservative calculation of the protection level in the baseline ARAIM algorithm. In this method, the equations for protection level calculation are derived at the theoretical level, and the maximum value minimization problem is transformed into an nonlinear constrained optimization problem to achieve a reasonable allocation of intactness and continuity risks, which increases the availability of ARAIM. Experimental results of the station data show that the average optimization rates of the horizontal protection level (HPL) and vertical protection level (VPL) for the BDS+GPS system are 50.61% and 46.88%, and the coverage of the BDS+GPS system with an ARAIM global availability greater than 99.5% increases from 85.53% to 97.37%, which is an improvement of 11.84%.