A precise thermal simulation model for distributed feedback Bragg (DFB) laser transmitter optical swb-assembly (TOSA) with uncooled transistor outline package is established based on the finite element method (FEM). The thermal gradient and junction temperature are simulated and analyzed. The self-consistency of this model is verified by comparing with experiments. The change of lasing peak wavelength as a function of case temperature is measured by pulse injection and the relationship is calculated. The relationships between laser junction temperature, case temperature of transistor outline (TO) case and thermal power are obtained by measurements. The laser junction temperature and TO case temperature are also simulated at different thermal powers. The simulated results are agree well with the measured results. The thermoresistance of the laser subassembly is calculated based on the measured temperature gradient at different environmental temperatures. The thermal model reported by this paper is very precise and can be used for thermal design of TOSA and transceiver.