The precipitating behaviours of phases in alloy 690 affect intergranular corrosion resistance significantly. The effects of chemical composition and heat treatment temperature on equilibrium phases in alloy 690 were simulated with Thermo-Calc software. The content of C is found to affect precipitating behaviour of carbide intensively and those of α-Cr and γ′ phases slightly. The content of Cr affects the precipitating behaviour of these phases markedly. The content of Fe has little effect on the precipitating behaviour of equilibrium phases. Solution treatment temperature should be above 1050 ℃, and thermal treatment temperature should be above 700 ℃