Based on the particle dense packing theory, the composition of cementitious materials and particle gradation of lightweight aggregate in ultra high performance concrete were determined. On this basis, the basic mix proportion of ultra high performance concrete with lightweight aggregate(LUHPC) was optimized. The effects of particle shape, pre absorption ratio and content of lightweight aggregate on the workability and mechanical properties of LUHPC were studied. Besides, the mechanism of the effect of lightweight aggregate on the properties of LUHPC was investigated by ultra depth field 3D digital microscope and scanning electron microscope. The results show that particle shape and pre absorption ratio are the key factors that determine the workability and mechanical properties of LUHPC. The workability and mechanical properties of LUHPC with spherical lightweight aggregate are superior to UHPC with crushed lightweight aggregate. With the increase of pre absorption of lightweight aggregate, the workability of LUHPC is improved, but the compressive strength and splitting tensile strength of LUHPC increase first and then decrease. With the increase of lightweight aggregate content, the workability and mechanical properties of LUHPC are improved first and then reduced. In addition, lightweight aggregate mainly improves the workability and mechanical properties of LUHPC by improving the sphericity of aggregate, the adhesion between lightweight aggregate and cement matrix and by providing internal curing effect.