Widely used in croplands, plastic mulch can significantly change land surface properties and energy partitioning. However, the magnitude of these modifications caused by plastic mulch (and its variations) on leaf area index remain largely unclear. Field experiments were, therefore, conducted to analyse the differences in energy partitioning between plastic mulch and non-plastic mulch conditions in cotton fields in arid Tarim Basin. Each component net radiation, surface soil heat flux, sensible heat and latent heat was either measured or estimated at different growth stages of the cotton crop. Results showed that the effects of plastic mulch on field energy partitioning was most evident when leaf area index was less than 1.0. During this period, net radiation decreased mainly due to the increase of surface reflectance. Surface soil heat flux and sensible heat were also increased due to the increase of surface temperature. Finally, latent heat decreased after plastic mulch application. As over 20% of net radiation was allocated to the soil surface under plastic mulch at the seedling stage, this suggests that surface soil heat flux should not be ignored for evaluating surface energy balance at the seedling stage under plastic mulch conditions.