The invention discloses a cross well cementation method without occupying a well mouth. The method comprises the following steps: (S1) the lower end of a 20" surface layer sleeve is connected with a sleeve float shoe, wherein the sleeve float shoe is provided with a side outlet hole on the side wall below a float shoe check valve base; (S2) the 20" surface layer sleeve is directly descended to a base, and is fixed in center through righting blocks in the well mouth; (S3) a drilling table inverts and swings the 20" surface layer sleeve in the position of conveniently connecting with a circulating head of the 20" sleeve below a turntable to shift a derrick to a next well position to perform the cross well cementation operation without occupying the well mouth in the well mouth area; and (S4) the return condition is observed in the well cementation period; and after cement is returned out from the well mouth, the replacement operation is immediately switched to finish the well cementation. The method finishes the well cementation operation by connecting the circulating head in the well mouth area, finishes the 20" surface layer sleeve well cementation operation without occupying well mouth time, finishes the all-well-section sealing by returning cement to the well mouth according to high-temperature and high-pressure shaft completeness requirements, and guarantees the integration of the 20" sleeve and a 30" pile pipe to realize the shaft completeness.