The invention discloses a construction method of a foundation pit of an underground diaphragm wall foundation and a foundation pit excavation system, and relates to the field of foundation pit construction. The construction method comprises the following steps that S1, a high-pressure water pump is arranged outside the foundation pit, a slag discharging pipeline and a water supply pipeline are arranged, a slurry boat is arranged, a gravel pump is installed, a construction platform is erected in the foundation pit, a relay gravel pump is installed on the platform, a slag discharging pipeline and a water supply pipeline are arranged at the pit bottom, and a slurry pump and a water jet gun are installed; S2, the water jet gun is used for conveying out high-pressure water to cut, disperse andexcavate a soil body in the foundation pit, and a mud and sand mixture is formed; and S3, the mixture is pumped and discharged by the bottom slurry pump to the gravel pump arranged in the pit, then the gravel pump in the foundation pit conveys the mixture to the gravel pump arranged outside the foundation pit, and the mixture is finally conveyed out by the gravel pump arranged outside the foundation pit through the slag discharging pipeline. According to the construction method of the foundation pit of the underground diaphragm wall foundation, due to the fact that a water supply system and aslag discharging system are arranged, the environmental protection requirement that the raising dust pollution is effectively controlled in the process of soil excavation is met, large-scale hoistingoperation is avoided, and the construction risk is reduced.