The invention relates to a tunnel construction method based on a fabricated support structure. A space grid structure is used as initial support of a tunnel, and the tunnel construction method includes the specific steps that S1, a full section excavation method is adopted to excavate the tunnel in sections; S2, a support structure of the space grid structure is assembled within a first preset length L<1>; S3, after the construction of the tunnel of the first preset length L<1> is completed, the next section of the tunnel is further excavated; and a support structure within the length of L<2>is assembled; S4, the two adjacent support structures within the preset length are connected longitudinally; S5, the next section of the tunnel is excavated after the construction of the tunnel of the preset length L is completed; a support structure within the length of Li is assembled; longitudinal connection parts between pieces are adopted to connect the head and tail of the twoadjacent support structures within the preset lengths longitudinally, wherein 2 tunnel; and S6, the S5 is repeated until the n preset length L, the support structure of the space grid structure is assembled.