The invention belongs to the technical field of high-proportion new energy power system stability judgment, and particularly relates to a method for determining the maximum replaceable capacity of a conventional power supply replaced with a new energy source. The method comprises the following steps: simplifying a power system; establishing a node voltage equation for simplifying the power system;establishing a node voltage equation after a system reference node is removed; removing a conventional unit part on the basis of the node voltage equation, thereby obtaining a relation between a system admittance element and new energy injection power; according to admittance properties and network transformation, deducing a simplified relation between the system admittance element and the new energy injection power; and by judging the positions of variables in the simplified relation in a complex plane, deducing conditions that the relation is established, thereby obtaining the maximum replaceable capacity of the conventional power supply replaced with the new energy source. According to the method, the critical conditions of system instability after the conventional power supply is replaced with the new energy source can be accurately deduced; the maximum capacity of the conventional power supply replaced with the new energy source can be quickly determined; and therefore, the method has relatively high application values.