The present invention relates to an under trimming device mounted below a needle late of a multi-needle sewing machine which mainly comprises an under trimming subassembly and a thread-sucking subassembly. The under trimming subassembly comprises a linkage and a thread-releasing means both being capable of shifting the under trimming subassembly into a standby position, extended position, thread-catching position, retracted and cut position, thread-clamping position, and a thread-releasing position in a series of linear movements. The thread-sucking subassembly is provided at one lateral side of the under trimming subassembly such that the looper thread cut and released by the under trimming subassembly can be instantly sucked and clamped in place and then used in the next sewing operation.