In order to reveal the non-uniform distribution of grain size in thick direction for engineering heavy plate, microstructure of 40 mm-thick Q345 steel was observed and measured under different short-term high temperature environments formed by fire. Moreover, the influence of the short-term high temperature environment was revealed on the distribution of ferrite grain size in the Q345 steel. Under different fire service environments, there was a log-normal distribution relationship between the distribution parameter N_f (number of ferrite grains) and d_f (average grain diameter), as well as ρ_Af (area fraction density) and d_f, at different positions along the thickness direction. However, the statistical results are greatly affected by the length of the statistical interval. When df is about 4 to 6 times the length of the statistical interval, the statistical accuracy is higher. By using nonlinear fitting method, multiple non-uniform distribution empirical models including N_f-d_f empirical formulas and ρ_Af-d_f empirical formulas were established at different positions along thick direction under various fire environments. Furthermore, the interrelationships between fire temperature T and N_f , T and ρ_Af , fire duration t and N_f , t and ρ_Af were revealed, respectively.