The influence of aggregate size effect on the improvement effect was revealed through the stirring and crushing test of expansive soil aggregates improved by industrial waste slag and the durability test of crushed soil， and the influence mechanism was analyzed. The results show that adding iron tailings sand and calcium carbide slag can accelerate the crushing of aggregates in the agitation process. The improvement effect of soil inside large particle size aggregates is poor， and cracks are prone to occur. The higher the content of aggregates larger than 15 mm， the poorer the durability of the samples. Compared with soil modified by calcium carbide slag， composite modified soil has better durability. For soil improved by calcium carbide slag and composite improved soil applied to the subbase layer， it is recommended to control the content of soil aggregates larger than 15 mm to be less than 2.5% and 21.8% respectively.