Combined with Chinese luminous efficacy model, this paper gets the annual and monthly total illuminance values of the natural light based on the solar radiation data of 54 consecutive years in Beijing. In this paper, the trends of annual and monthly total illuminance levels in Beijing were analyzed by Mann-Kendall trend analysis and Mann-Kendall mutation test. The results showed that the basic trend of annual total illuminance values in Beijing was significantly decreasing and the declining trend became more evident after mutation occurred in 1977. The basic trends of the monthly total illuminance values were significantly decreasing. The monthly illuminace appeared to decline most steeply in July. In contrary, the least decrease happened in November. Only mutations in May happened in 1967. In other months, mutations occurred between 1975 and 1981. The fluctuation of the trends of monthly illuminace values was not apparent before mutation. The declining trend of monthly total illuminance values significantly increased after mutation. The steady decline of annual and monthly total illuminance values in Beijing can be primarily attributed to the aerosol optical thickness caused by cloudiness and atmospheric pollution.