Through the test of mechanical property of cement mortar ,the paper carried out the flexural, compressive strength test of cement mortar mixed with different fineness of limestone powder in different curing days under the condition of a certain dosage of steel slag. The results show that under the condition of a certain dosage of steel slag,compared with cement mortar that is not added to limestone powder,the addition a certain amount of limestone powder can improve compressive and flexural strength of cement mortar;under the condition of a certain dosage of steel slag, with the increase of fineness of limestone powder, the ratio of compressive strength to flexural strength decreases, the flexibility of the cement mortar becomes better and better. The paper simulated the ratio of compressive strength and flexural strength of cement mortar by use of MATLAB curve fitting toolbox.The result indicated that the fitting of ratio of compressive strength to flexural strength simulated by a exponential function is better.