As the deviation between the real parameters and the thermal design parameters of a space spectral imaging apparatus in orbit effects on the design accuracy of complete appliances, this paper analyzes the thermal design based on the system sensitivity theory, and establishes the heat balance equations in orbit. Under the design variable analysis of the heat balance equations, thermal design parameters effecting the temperature distributions of complete appliances are given. As an example, the sensitivity of thermal design parameters mentioned above is analyzed for the space spectral imaging apparatus. The analytical results show that the sensitivity of solar absorption coefficient to the mean temperature is equal to zero, and the sensitivities of emissivity , inner heat source, thermal conductivity, and contact thermal conductivity to the mean temperature are 2.2-14.55 ℃, 1.8-2 ℃/W, 2.25×10-3- 4.39×10-2 m℃2/W and 0-1.1×10-3 m2℃2/W, respectively. Test results prove that the sensitivity analysis based proposed thermal design scheme is effective and feasible proved by thermal test results.