In the shipping industry of the Yangtze River, ships have a significant power demand. As the cleanest green energy, the photovoltaic (PV) system can substantially reduce reliance on traditional fuel oil, thereby mitigating carbon emissions and promoting environmental conservation. However, due to the dynamic changes in longitude and latitude as well as the ship's motion during the voyage, the output power of the ship's PV system exhibits high-frequency fluctuations. Considering the uncertainty of ship sailing plan and the difficulty of PV data collection, a modified clear sky model is proposed in order to improve the calculation accuracy of PV power during inland vessel sailing. In this modified clear sky model, the ship's navigation attitude and location are integrated into the mathematical model of marine PV system as important factors. The validity of this model is verified by Python simulation calculation. The results demonstrate that the R square value of the calculation model is not less than 0.989.