Grid-forming converters has attracted increasing attention, whose stable transmission of maximum active power under complex grid conditions is important to the enhancement of power generation capacity. This paper proposes a quantitative analysis method of the active power transfer limit under the resistive-inductive grid condition, considering both power flow constraints and grid-forming control constraints. First, the limiting factors of the maximum active power is discussed, including power flow constraints (e.g., voltage/current amplitude limits) and grid-forming control constraints (e.g., virtual active/reactive power control limits), from which a set of systematic operating region conditions are constructed. Second, a geometric method for the maximum active power is proposed, providing an optimal active power reference under active-reactive power coupling, with consideration of both power flow and grid-forming control constraints. Finally, the correctness of the proposed method is validated on a real-time simulation platform, considering different short circuit ratios (SCR), resistance/inductance (R/X) ratios and main control parameters.