The effects of different strain rates and cathodic protection potential on the stress corrosion cracking (SCC) behavior of new-developed 1200 MPa Ti-35421 (Ti-3Al-5Mo-4Cr-2Zr-1Fe) alloy have been investigated by using slow strain rate test combined with in-situ electrochemical monitoring. The results indicate that Ti-35421 alloy has the highest SCC susceptibility in 3.5% NaCl solution at the strain rate of 1.67×10-5 mm s-1. The plastic loss and stress corrosion index of Ti-35421 alloy are confirmed to be 27.27% and 0.273, respectively. The passivating ability of the passivation films in the crack tips is considered to be weaker than active dissolution under the combined interaction of applied stress and corrosive media especially with strain rate of 1.67×10-5 mm s-1, which results in the intensification of SCC. When the cathodic protection potential is set at -600 mV, the SCC susceptibility of Ti-35421 alloy in 3.5% NaCl solution is the lowest. The range of the optimal cathodic protection potential is measured to be from -450 to -600 mV. The reduction of the plastic loss and stress corrosion index of Ti-35421 alloy down to 1.01% and 0.113 is ascribed to the slower conjugated anodic reaction kinetics under the cathodic protection.