In unsteady state heat transfer processes， existing theories still cannot effectively solve the problem of initial heat flux density values. This article proposes the concept of maximum contact heat flux density and the contact heat flux density constant. Based on this， a calculation formula for the maximum contact heat flux density is proposed， which determines that the maximum contact heat flux density is proportional to the initial temperature difference between the two objects， and the proportionality coefficient in the formula is the contact heat flux density constant. Additionally， based on the experimental principle of approximating instantaneous heat flux with an extremely short average heat flux density， a heat flux testing platform is established to conduct temperature rise tests on metal sheets. The maximum contact heat flux density and contact heat flux density constant are calculated. Based on experimental research， the contact heat flux density constant is 1 022.51 W/（m2·K） for stainless steel 304 metal sheets with a thickness of 0.5~5.0 mm and a temperature rise of 100 K.