A direct drive wind farm (DDWF) is composed of multitudinous permanent magnet synchronous generators (PMSG), with a small single-unit capacity. When the power grid mal-functions, some units in the DDWF may enter a fault traversal state, and some units may be disconnected from the grid. At this time, the DDWF exhibits a decrease in active output. Due to the large number of units in DDWFs, it is difficult to directly monitor each off-grid unit, resulting in the inability of the power grid to timely and accurately determine the load shedding amount. This article proposes a monitoring method for the disconnection capacity of DDWFs, which can calculate the disconnection capacity of DDWFs after a fault occurs by only monitoring the electric quantity at the connection point of the DDWF. And a DDWF grid connection model is built using PSCAD simulation software. The reliability of the monitoring is authenticated through simulation using a single-phase grounding short circuit fault as an example.