The invention relates to a preparation method of a tin based nano composite material for detecting low concentration acetone gas, and aims to solve the problems that in the prior art, a gas sensitive material for detecting acetone gas does not respond to low concentration acetone gas, the sensitivity on high concentration acetone gas is low, the responding speed is slow, and the selectivity stability is bad. The composite material comprises the following raw materials: stannous chloride, sodium chloride, sodium citrate, and praseodymium nitrate, and is prepared by the following steps: hydrothermal synthesis in a reactor, microwave heating high temperature heat treatment, grinding, and sieving. The technology is advanced, the data is precise, the product morphology is good, the product has a sheet flower-shaped layered structure, the sheet thickness is not more than 30 nm, the product purity is good and can reach 99%; the sensitivity of the composite material on acetone gas (100 ppm) can reach 27, the responding time is 2 seconds, the recovery time is 36 seconds, the sensitivity on acetone gas (1 ppm) can reach 1.86, the responding speed is quick, the composite material is suitable for low concentration acetone detection, and can be used to prepare sensors for detecting acetone gas, and the provided preparation method is advanced.