According to the situation of low unif ormity and large thickness of direct-type LED back light module (BLM),optical s imulations based on nine-value method,combined with TracePro are performed to investigate optical properties of secondary lightguide-diffusive component in direct-type LED BLM.Effects of incident surface,exit surface,height and thickness of seconda ry lightguide-diffusive component on optical performance of direct-type LED BLM are studied theoretica lly.Simulations reveal that the uniformity of direct-type LED BLM is mainly dependent on the in cident surface and exit surface of the secondary lightguide-diffusive component.The uniformity is incr eased when the incident surface is depression structure and exit surface is bending structure,and the uniformity has slight increase as radiu s of depression structure decreasing and radius of bending structure increasing, respectively.The height and thickness of the component can influence the facula display effect of direct-type LED BLU.The boundary o f facula is slightly expanded and the dark space group is shrinked as the height of component increases.The illumination maximum and number of lightspots increa se as the thickness of component increases, respectively.Then the uneven light and dark phenomenon happen,which affect the b ack light subjective effect.