Abstract: Thermoset polyimide resins with molecular weight of 1 500, 2 500, 3 500, 5 000 g/mol were synthesized firstly by asymmetric dianhydride 2,3,3',4'-biphenyltetracarboxylic dianhydride (α-BPDA), diamines and 4-phenylacetylene anhydride (4-PEPA) as the endcapping agent. Then, the relation between molecular weight and curing reaction process, aggregation structure, heat resistance, rheology property and toughness property, and its mechanism were investigated by characterisation and analyses methods such as FTIR, XRD, DSC, DMA, TGA, rheometer and so on. The results indicate that with the increase of molecular weight, glass transition temperature of resin decreases from 370 ℃ to 344 ℃, the lowest melt viscosity increases from 15.5 Pa·s to 37 090.0 Pa·s and impact strength also increases from 25.93 kJ/m² to 45.04 kJ/m².The glass transition temperature and processability of the thermoset polyimides will reduce by increasing its molecular weight, but toughness will increase.