To explore the effects of lightweight sand on the strain hardening properties of ultra high performance concrete (UHPC) with different specimen sizes, the lightweight sand was used to replace the yellow sand by equal volume. Nine groups of uniaxial tensile tests were carried out with different lightweight sand volume rate ranging from 0 to 35% and different specimen thicknesses from 30 mm to 100 mm. Meanwhile, simultaneous acoustic emission real-time flaw detection tests were conducted. Results show that the volume rate of lightweight sand demonstrates little effect on the stress and strain at the elastic limit point of UHPC, but when the volume rate of lightweight sand increases from 0 to 35%, the ultimate tensile strength and ultimate tensile strain of UHPC grow from 10.6 MPa and 2.35×10^-3 to 19.4 MPa and 4.3×10^-3 respectively. When the volume rate of lightweight sand is greater than 15%, the strain hardening degree of UHPC significantly increase with more damage points generated and more uniformly distributed inside the specimen, showing remarkable crack control capability. With the same lightweight sand volume rate, the strain hardening degree of UHPC decreases with the increase of specimen thickness, and the damage points inside the specimen tend to be concentrated, exhibiting an obvious size effect.