A traditional average-atom model (AAM) is improved in several aspects. 1. A dynamic criterion for free electrons is adopted. 2. The free electrons are classifie d in three types, each of which is treated by the partial wave method and Fermi- Dirac statistics. 3. An exchange potential in mixed type is obtained to replace the Slater potential by applying the variation principle. 4. A parameter α in t he atomic structure is calibrated by the constants of material under normal cond itions. As examples electron pressures of Iron, Nickel and Copper at zero temper ature are calculated and compared with the results obtained by other theories an d in experiments.