The invention provides a two-stage piston expander. The two-stage piston expander comprises an I-stage expanding mechanism, an II-stage expanding mechanism, a crankshaft and a pipeline. The I-stage expanding mechanism is connected with the II-stage expanding mechanism through the pipeline. The I-stage expanding mechanism comprises an I-stage air inlet valve, an I-stage exhaust valve, an I-stage piston, an I-stage expanding cylinder and an I-stage connecting rod. The II-stage expanding mechanism comprises an II-stage expanding cylinder, an II-stage exhaust valve, an II-stage piston and an II-stage connecting rod. A one-way valve is arranged on the pipeline. An inlet of the pipeline is connected to the bottom of the I-stage expanding cylinder. An outlet of the pipeline is connected to the top of the II-stage expanding cylinder. The other end of the I-stage connecting rod and the other end of the II-stage connecting rod both are connected to the crankshaft. According to the two-stage piston expander, the problems that when the ratio of the air inlet pressure to the environment pressure exceeds the single-stage piston expansion ratio, the exhaust pressure is too large, incomplete expanding is caused, and the loss is large are solved through the twos-stage expanding mechanisms, and the output efficiency and the energy utilization efficiency of the piston expander are improved.