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Classic linear models of fossil fired steam units have been used for decades in power system dynamic analyses. The effect of main steam pressure is often ignored or the more complex models of boiler and its control systems should be adopted in the simulations. Moreover, the variations of turbine enthalpy drop are not considered in those models, which may result in a great deviation of output power during a transient period after a disturbance. A new model with little increase in complexity is built in this paper to improve the accuracy of classic models, where the boiler effect and the enthalpy droop variations are considered by a simplified boiler model and replacements of three turbine model parameters. Test data of a 600-MW coal fired generation unit are adopted to compare the performance of the proposed model with that of a classic one. Higher accuracy of the new model are observed and it is proposed to replace the classic models in power system dynamic analyses.