A novel direct circulation well cementing device and a well cementing method for a high-pressure gas underground gas storage well are characterized in that the well cementing device is composed of a bottom seal head (1), a stainless steel plug (2), a plug press rod (3), a second plug (4), a casing joint (5), a sealing ring (6), a bolt (7) and a well cementing connecting head (8); the second plug (4) is sleeved at the outside of the upper end of the plug press rod (3), and the casing joint (5) is sleeved at the outer circle of the upper section of the bottom seal head (1). The invention realizes the dual-sealing and leads the sealing to be more reliable and safer, and the first sealing adopts the cone sealing and the second sealing adopts 3-5 time sleeve button sealing. When in well cementing, a connecting well cementing pump pours cement slurry in, and the cement slurry enters space of an outer ring of a gas storage steel pipe from a hole (11) formed at the bottom part of the bottom seal head (1) to fill up the whole space of the outer ring, thereby realizing the direct circulation well cementing.