Abstract: A frequency step-scanned IPDA lidar was designed for space-borne CO<sub<2</sub< detection. Compared with single on-line wavelength IPDA lidar of ESA, the distributed wavelength sampling across the line region could reduce the impacts of wavelength dependent responses on the lidar and also reduce the relative error. The echo signal, relative random error, frequency drift error and pressure uncertainties error as a function of the on-line wavelength with one scanned cycle was simulated. The total CO<sub<2</sub< concentration relative error is 0.084 2% with 0.035 sr<sup<-1</sup< reflectivity, 0.3 MHz frequency drift error and 0.001 pressure uncertainties error when the on-line wavelength stepped in 6 361.235 0 cm<sup<-1</sup<. The results show that compared with 1.0×10<sup<-6</sup< accuracy of single on-line IPDA lidar, the measurement error of space-borne CO<sub<2</sub< wavelength step-scanned IPDA lidar is less than 0.5×10<sup<-6</sup<.