Slim hole grouting well cementing method and device

The invention discloses a slim hole grouting well cementing method and device, and belongs to the technical field of well drilling. The method comprises the steps that (1) after well drilling is completed, the lower end of a drill rod is connected with a thin pipe through a reducer union, the drill rod and the thin pipe are placed into a to-be-sealed section of a slim hole, the lower end of the thin pipe is made to reach the position below the to-be-sealed section, cement paste is injected into the upper end of the drill rod, and the cement paste can flow to the to-be-sealed section from the thin pipe; (2) after grouting is completed, an extrusion plug inserted with a sealing core is added into the drill rod, the extrusion plug is in sealed connection with the inner wall of the drill rod, and water or slurry is introduced into the upper end of the drill rod to enable the extrusion plug to push the cement slurry downwards; when the extrusion plug reaches the lower end of the drill rod, water or slurry cannot b