Regarding the active power transfer limit issues for new energy units in weak power grids, current research lacks analytical methods considering multiple electrical constraints of new energy systems, including constraints such as maximum steady-state operating current and the voltage variation range at the grid connection point. To address this, the static voltage feasible region under multiple electrical constraints is characterized through the counter lines of electrical quantities, and the analytical expressions for the active power transfer limit under multiple electrical constraints are derived accordingly. Next, the mechanism of active power transfer limit for new energy converters under weak power grids is revealed, where the influence of parameters and the Q-U characteristics is studied. Following this, to satisfy the requirement of operate in weak gird condition within the rated generation range, the minimal parameter requirement is investigated, and the optimal Q-U droop coefficient range is derived to ensure the constrains are always satisfied  Finally, the correctness of the power transfer limit function and the effectiveness of the proposed improvement method is verified through the simulations on PSCAD/EMTDC platform in an extremely weak power grid with a short-circuit ratio of 1.05.