The entry-pupil radiance of the optical imaging instruments of Tianwen-1 varies considerably during its orbit operation， owing to the changes in the solar elevation angle and ground scene reflectance. To achieve the best imaging effect， the optical imaging instruments should utilize on-orbit adaptive adjustment gain. The solar elevation angle is an important parameter that is used to set the integral series of the time delay and integration （TDI） charge-coupled devices （CCDs）. Furthermore， the integral series is the main parameter used to adjust the gain. This study presents a method for calculating the solar elevation angle in real time at the sub-satellite point of the Mars orbiter based on Fourier fitting， to mitigate the challenges of the real-time variation in the solar elevation angle and the large ephemeris file created during the orbiting period. First， an 8-order Fourier approximation based on the principle of least squares is utilized to fit the x， y， and z coordinates of the sun vector in the Martian inertial coordinate system， and a fitting equation is obtained as a function of time. Second， the real-time coordinates of the orbiter in the Mars inertial coordinate system are obtained based on the orbit parameters sent by the guidance and navigation control system. Finally， the solar elevation angle of the sub-satellite point can be calculated in real time on the orbit based on the cosine formula of the included angle. The experimental results show that the maximum absolute error of the real-time calculation results of the solar elevation angle obtained by using this method is less than 0.3° during the period from 2021-01-01 00：00：00 UTC to 2024-01-01 00：00：00 UTC. The accuracy requirements of the calculation results of the solar elevation angle of the TDI CCD integral series of the Tianwen-1 high-resolution camera are satisfied. Based on this method， the Mars image obtained by the high-resolution imaging camera of Tianwen-1 shows rich details with reasonable brightness and contrast.