Solar Irradiance Monitor (SIM) on the FY-3C satellite was designed to be a Small-field-of-view Radiometer (SFoV-R) with a precise solar pointing system to decrease the measurement uncertainty derived from the wide field-of-view of SIM on FY-3A and FY-3B satellites. To obtain the measurement uncertainty derived from the SFoV-R, the field-of-view of SFoV-R on FY-3C satellite determined by a view-limiting aperture and a precision aperture was calculated. Based on this field-of-view, a theoretical change curve of the measured irradiance incidence angle was achieved. Then, an actual measurement of the change curves was performed. The result indicates that the relative deviation on an average between the theoretical and measured change curves is 1%-2%, which is close to the limit accuracy of the measuring installation and proves the veracity of the calculated values for field-of-view. The correction term of radiation exchange between space and the radiometer is estimated to be 2.023 W/m2 based on the calculated field-of-view. Experiments show that the standard deviation of this correction term can be neglected compared to the requirement of solar irradiance measurement accuracy.