Abstract: The east-west ridge cultivation technique can effectively improve the mechanical operation efficiency in the solar greenhouses. However, the generally dimmed canopy can be found in the north side of the solar greenhouse and the north side of each ridge, compared with the south side. This study aims to improve the light environment inner tomato canopy, particularly for the better growth and development of the cultivated crops. A theoretical method was proposed to hang the diffused reflective film on the back slope of the solar greenhouse, according to the geographical location and building parameters. The appropriate angle was also determined for the diffused reflective film on the light environment of the greenhouse. The field test was conducted in the solar greenhouse of Hongke Farm (115.97E, 39.62N) in Beijing of China in February of 2022. Four east-west ridges were set in the experimental greenhouse. The greenhouse was separated as an experimental and control section. Among them, a diffuse reflective film with an appropriate angle was hung on the back slope of the solar greenhouse. The tomato plants were managed in the experimental and control sections, according to the normal horticultural practice. The light sensors were placed horizontally upward to measure the light intensity from above at the 1.0 and 1.4 m height of the canopy on the north side of the ridge, respectively. By contrast, the sensors were placed vertically backward to record the light intensity from the north side at 1.0 and 1.4 m height of the canopy. The results showed that the effect of the diffuse reflective film on the canopy light intensity was relatively low in the morning, due to the relatively low light intensity and the greater sun azimuth. At noon, the application of diffuse reflective film was improved the light intensity from the north of each ridge, compared with the control, while the light intensity from the north at 1.0 and 1.4 m increased by 52.4% and 68.8% for the third ridge, respectively. The overall light intensity at the 1.0 m height of the canopy increased for all the ridges. At the height of 1.4 m, only the light intensity of the third ridge at the experimental section was significantly higher than that of the control, and the light intensity increased by 27.1%. In the afternoon, the light intensity from the north of each ridge also increased, and the third ridge increased the greatest, while the light intensity from the north direction increased by 69.2% and 102.0%, respectively, compared with the control. The light intensity of the incoming light from above of the second, the third and the fourth ridge were all significantly increased, and the fourth ridge was resulted in the greatest improvement, indicating both the optimal heights of 1.0, and 1.4 m (46.0% and 54.3%, respectively). Therefore, the diffused reflective film can be expected to increase the light intensity of the inner canopy, providing for the incoming light from the north side. As such, the illumination uniformity of the greenhouse can increase to improve the light environment in the greenhouse.