LED operation life is directly reduced by very high operation temperature and the luminous intensity as well as efficiency are affected, leading to the optical characteristics of large-size LED backlight reduced greatly. From side LED backlight module, IcePak software is designed and used to simulate the influence on backlight module temperature from the width and thickness of heat-dissipation Aluminum substrate. And the heat-dissipation structure suitable to large-size LED backlight module is given. The results show that with the increasing of the length and thickness of Aluminum substrate, the temperature of LED Bar will reduce. When the Aluminum substrate with 200 mm length and 2 mm thickness is chosen, the temperature of LED Bar is 69.4 ℃, which is less than 70 ℃ and the design requirements are met and the lowest cost is got. LED Bar temperature of the prototype is measured and the highest temperature is 69.7 ℃, which is much closed to simulation results. The structure has simple manufacture technology and low cost and accords with the requirements of light weight and thinner backlight module, which has a certain market value.