X70 pipeline steel might be used in deep sea for oil exploitation. Therefore, it is necessary to evaluate the security of service of X70 pipeline steel in a deep-sea environment. Stress corrosion cracking (SCC) are the primary influences on the life. The effect of high hydrostatic pressure (HP) on SCC threshold stress is far from clear. In this paper, a sealed autoclave with an additional loading device was designed to examine in a simulated deep-sea environment by constant load testing, electrochemical measurements, confocal scanning laser microscope analysis and SEM observation. The results revealed that the threshold stress for SCC is 0.89σb, 0.78σb and 0.68σb in 3.5% NaCl solution at 0.1MPa, 5 MPa and 10 MPa, respectively. When the solution acidity increases to pH=3, the threshold stress decrease to 0.83σb, 0.68σb and 0.53σb, respectively. The results indicate that HP promotes SCC, particularly in acid solution.