Although future reduction of the global frequency of tropical cyclones is known to be a robust response of dynamical numerical projections to global warming, its mechanism is not yet fully explained. We propose a diagnostic relation based on the convective mass flux to constrain the global frequency of tropical cyclones. Simulation results using a high-resolution global non-hydrostatic model showed that the reduction in the global frequency is much larger than that in the total tropical convective mass flux. Either a future increase in the frequency of stronger tropical cyclones or an areal increase in strong updrafts explains this difference. A reduction in the contribution of convective mass flux of tropical cyclones to total tropical convective mass flux also contributes to the difference. This study suggests that future intensification of tropical cyclones leads to future reduction in their frequency under the condition that the contribution of tropical cyclone remains the same or smaller.