A new design of planar mirror multipass cell (MPC) is proposed. In this scheme, the new MPC consists of two low-cost silver (or gold)-coated plane mirrors and two biconvex lenses, so compared with the traditional expensive concave mirror and cylindrical mirror MPCs, which is more economical and applicable. In the new MPC, the outer side of the plane mirror is coated with high reflectivity silver or gold film. This coating method insulates the reflective coating of the plane mirror from the corrosive target gas, reduces the pollution of the corrosive gas to the protective silver (or gold) layer of the plane mirror, so as to improve the service life and detection accuracy of the instrument. In order to verify whether the new optical multipass cell has the capability to provide an effective long optical path, the optical simulation of three new optical MPCs with different design parameters is carried out by using optical software. Compared with the traditional White cell and Herriot cell (about 700 cm3 in volume, with about 80 times reflections), the simulation resultsshow that 62, 100, 99 times of light reflections and the effective optical path of 14.64, 40, 42.57 m can be achieved in a small volume (about 236, 393, 422 cm3). The ratio of the effective optical path to the volume of the multipass cell is 100, 62 and 101 mm－2 respectively, which indicates that the space utilization of the new optical MPC is very high. It is shown that this new type of optical MPC has the advantages of small volume, long optical range, low cost and high sensitivity, which is expected to be used in environmental monitoring, combustion process analysis, medical diagnosis and other fields due to the high detection sensitivity to trace gases.