The invention relates to the field of a lithium-ion battery and specifically discloses a lithium-ion battery diaphragm with a promoted heat resistance, a preparation method thereof and a lithium-ion battery. The method comprises the following steps: (1) melting and blending polyethylene, cross-linking agent and cross-linking auxiliaries, thereby acquiring a mixture A; (2) molding the mixture A into an initial diaphragm B, wherein the gel content of the initial diaphragm B is less than 5wt%; (3) performing high-temperature cross-linking on the initial diaphragm B, thereby forming a cross-linking diaphragm C, wherein the gel content of the cross-linking diaphragm C is 30-70wt%; (4) stretching the cross-linking diaphragm C, thereby acquiring a stretched diaphragm D; (5) annealing the stretched diaphragm D, thereby acquiring the lithium-ion battery diaphragm, wherein the melt index of polyvinyl is 0.02g/10min-5g/10min, and in 100 weight parts of polyvinyl, the dosage of the cross-linking agent is 0.03-8 parts by weight and the dosage of the cross-linking auxiliaries is 0.03-10 parts by weight. The method can be adopted for preparing the lithium-ion battery diaphragm with promoted heat resistance.