An apparently seamless lengthened bamboo sheet is formed by lengthening a plurality of bamboo strips or blanks and assembling a plurality of lengthened bamboo strips or blanks together. A slot is formed on at least one end of the bamboo strips or blanks in direction of length, and a tenon corresponding to the slot is formed on the same end; the tenon on one bamboo strip or blank is embedded into the slot on another bamboo strip or blank. The tenon and slot are mutually matched and the two bamboo strips or blanks are engaged to each other. A method to manufacture is the apparently seamless lengthened bamboo sheet includes: forming a slot and a tenon on the end of bamboo strips or blanks; embedding a tenon at one end of one bamboo strip or blank into a slot on one end of another bamboo strip or blank; assembling and gluing a plurality of lengthened bamboo strips or blanks in direction of width or thickness; forming the bamboo sheet by a common process including pressing. The slot-tenon connecting structure of the present invention makes the seams on the surface unobvious and enhances the connection strength.