Based on the change law of solar radiation intensity and shadow of land bridge variation with time, the mathematical model expressing of the relationship among time, solar radiation intensity,solar position and shadow track was established. The effects of the sunshine-shield of land bridge floor were studied using the model. Direct rate concept was proposed to define the proportion of solar direct radiation energy of the surrounding permafrost. The results show that height, direction and width of land bridge are the major influence factors on the solar radiation of underbridge and surrounding permafrost. With the increase of the height, shading scope and direct rate of sunshade center also increase; with the increase of the width, direct rate of the surrounding permafrost decrease; there was obvious asymmetry distribution of permafrost surface direct rate, especially for the east-west bridge. Sunshine-shield of land bridge can effectively reduce the sun radiation heat of the surrounding permafrost and also cause non-uniform distribution of solar radiation on permafrost surface around piles, which should be concerned in the stability estimation of land bridge.