The invention discloses a water stop method for an upper-soft and lower-hard water-rich stratum foundation pit engineering retaining structure. The method includes the steps that firstly, a cast-in-situ bored pile is constructed; secondly, on the outer side of the cast-in-situ bored pile, a high-pressure jet grouting pile or a three-shaft stirring pile is constructed; thirdly, a hole is drilled in the center of the high-pressure jet grouting pile or the three-shaft stirring pile, and high-pressure jet grouting or sleeve valve pipe grouting is performed; fourthly, deep hole curtain grouting is performed; fifthly, a first-layer small guiding pit is excavated; sixthly, preliminary-stage supporting is performed on an upright excavation face of the first-layer small guiding pit; seventhly, core soil is reserved during excavation of a first-layer foundation pit; eighthly, a first-layer transverse steel purlin is arranged, and a steel cross brace is erected; and ninthly, when the pre-designed strength of the retaining structure is achieved and the monitored and measured result is accurate, next circulation is performed, and the fourth step, the fifth step, the sixth step, the seventh step and the eighth step are repeated till construction ends. The method is good in main water stop effect and good in flexibility. The technology is simple, the construction risk is small, influences on the environment are small, and adaptability to a complicated stratum is high.