The effect of fly ash content on the performance of seawater and sea sand high performance concrete was studied by carrying out experiments on mechanical properties, chemical shrinkage, pH value, chloride ion concentration, and microstructure. The results show that in order to maintain the stability of reinforcement passive film, the content of fly ash should not be greater than 30% (mass fraction, the same below) under high temperature steam curing and 50% under standard curing. The concentration of free Cl- in seawater and sea sand high performance concrete fluctuates with curing time, increases first and then decreases sharply in the early stage, and increases slowly in the later stage. The Cl- concentration under standard curing is significantly lower than that under high temperature steam curing. Seawater and sea sand high performance concrete has early strength, and its strength decreases with the increase of fly ash content. High temperature steam curing significantly improves the flexural and compressive strength of concrete. The more fly ash is added, the more unhydrated particles remain. High temperature steam curing effectively improves the microstructure and compactness of concrete. When fly ash content is too large or too small, it increases the chemical shrinkage of Portland cement. The chemical shrinkage value of concrete is small when the fly ash content is 30% and 40%.