The invention relates to a well cementing method for an ultra-deep narrow-gap tail pipe with a stratum intrusive body. The well cementing method comprises the following steps that (1), the tail pipe is put into a shaft to the to-be-cemented position, and the intrusive body in a well is discharged; (2), well cementing liquid is injected into the well, displacement is conducted in place, and when the well cementing liquid is injected, the construction displacement and the construction pressure are controlled according to a pressure window at the well depth where stratum intrusion occurs; and (3), shutoff-pressure reverse extrusion is conducted. According to the well cementing method for the ultra-deep narrow-gap tail pipe with the stratum intrusive body, the pressure window is judged firstly, balanced-pressure well cementing is conducted under the pressure window to ensure that a stratum is stably pressed, and intrusive body overflowing is reduced; and after mud displacement is finished,the intrusive body which is likely to go upwards is extruded into the stratum through the shutoff-pressure reverse extrusion measure, and the cementing quality of an overlap section is guaranteed. The well cementing method has the advantages that the ultra-deep narrow-gap tail pipe with the stratum intrusive body can be better cemented, the cementing quality is improved, and moving is effectivelyprevented.