The key to realize high precision positioning at sea is to obtain real-time precision satellite orbit and clock products based on statespace representation (SSR) format for real-time precise point positioning (PPP). Compared with Inmarsat communication services, BeiDou Short-Message Communication (SMC) has been widely studied to complete the transmission of SSR data in maritime application because of its low cost and global coverage. Although the custom coding technique has overcome the limitations of SMC in the broadcast frequency and bandwidth for SSR data transmission applications with precision up to centimeter level, the correction accuracy of SSR data will decrease while the transmission delay exceeds one minute because the broadcast success rate of SMC technology is 95%. In this contribution, we present a real-time PPP/inertial navigation system (INS) tightly coupled integration method with the BeiDou SMC to solve the issue of PPP-based positioning accuracy decrease, when the events of SMC broadcast failure occur. The PPP and IMU are tightly coupled to weak the effect of GNSS orbit and clock correction error by strong autonomy and position constraint of high-precision inertial measurement unit (IMU). Meanwhile, considering that the satellites orbit and clock variables have different correction accuracy when the SSR data delay add up to one minute, the robust Kalman filter based on IGGIII is derived to adjust the weights of GNSS observations, which effectively reduces the impact of the observations with abnormal correction. In order to verify the robustness of the algorithm to maintain the positioning performance when the SSR transmission of the BeiDou short message fails, we control the BeiDou short message broadcaster to achieve a minute-level delay to simulate the actual situation. The results show that when the transmission age of SSR exceeds about five minutes, the positioning accuracy decreases obviously. In the static environment, the positioning accuracy of PPP/INS is 4 %, 8 %, 10 %, 29 %, and 46 % higher than that of PPP when the transmission age of SSR is 2min, 4min, 6min, 8min and 10min. In the dynamic environment, the PPP/INS increases to more than 5% compared to PPP positioning when the transmission age of SSR exceeds 5 minutes. The experimental results show that the proposed method can effectively reduce the impact of SSR age growth caused by the low success rate and frequency of BeiDou short message broadcasting. This method improves the accuracy and stability of global positioning applications.