Using steel slag powder and fly ash to replace cement can promote the secondary utilization of industrial solid waste and protect the ecological environment. Besides, fly ash can save costs, improve workability and improve the later strength of concrete. Four groups of concrete test blocks were tested by the combination of the two to replace cement, and the concrete appearance and compressive strength of the test blocks in different periods were tested. The experimental results show that: 1. Under the premise that the total amount of composite substitution is 40%, increasing the content of steel slag powder can effectively improve the compressive strength of concrete, but excessive addition and substitution will make it counterproductive. 2. Steel slag is regarded as overburned Portland cement clinker, which can be used as auxiliary cementing material to improve the activity of cement, while fly ash can improve the workability of concrete and improve the later strength of concrete. 3. The recommended optimal composite substitution ratio is cement: steel slag powder: fly ash = 12:5:3.