The invention discloses a trenchless combined PDC reamer bit, and belongs to the technical field of drills. The trenchless combined PDC reamer bit includes a main shaft, a fixed disk and a plurality of reaming fins; the fixed disk sleeves the main shaft and is fit for the main shaft in an interference mode or fixedly connected with the main shaft, the multiple reaming fins are distributed around the circumferential direction of the main shaft, and the multiple reaming fins are detachably connected with the main shaft and the fixed disk; and a plurality of cutting teeth are arranged on each reaming fin. Through that the multiple reaming fins are detachably connected with the main shaft and the fixed disk, or the multiple reaming fins are detachably connected with the fixed disk, by changingthe reaming fins, changing new cutting teeth or changing different kinds of cutting teeth is achieved; on one hand, the efficiency of reaming holes is improved; and on the other hand, different kindsof the reaming fins just need to be carried in the process of the construction, the requirement for different strata can be met, and the construction cost is greatly decreased.