The invention relates to a random access channel assignment method of a medium coverage system and a large coverage system in a TD-SCDMA evolution system based on the OFDM. The random access preamble sequence is able to be assigned at the back end of the up time slot as possible under the condition that bigger interference exists by adding a guard gap in the random access preamble sequence, thereby the technical problem that a TS0 time slot and a DwPTS time slot of the distal cell generate interference to self-cell random access preamble sequence in the medium coverage system and the large coverage system. The assignment of the random access channel is more flexible by using the invention, and the detection success probability of the preamble sequence from the base station is improved.