A dual-mode pressure reducer, belonging to the technical field of pressure reducers and comprising: a sampling head lower housing (6) and a sampling head upper housing (1) connected to each other. The top of the sampling head lower housing (6) is provided with a petri dish holder (5) for fixing a petri dish. The top of the sampling head upper housing (1) is provided with a gas diversion blocking plug (2). The bottom of the sampling head upper housing (1) is provided with a gas diversion sieve plate (4). A gas pipe connector of the sampling head lower housing (6) is connected to a through-wall gas connector (17) via a pipe. A gas outlet end of the through-wall gas connector (17) is connected to a T-type gas connector (18). One end of the T-type gas connector (18) is connected to a pressure sensor (14), and another end is connected to a safety valve (7). A gas outlet of the safety valve (7) is connected to a gas inlet of a flow sensor (8). A gas outlet of the flow sensor (8) is connected to a pressure reducing
