The invention discloses a digital AC grid insulation monitoring instrument which solves a problem that insulation monitoring for the existing ship AC grid is low in accuracy and reliability. The digital AC grid insulation monitoring instrument comprises N current transformers, N low-pass filters, an AD converter, a switching value information input module, a switching value information output module, a first optical coupler, a second optical coupler, an information monitoring host and a PC (Personal Computer), wherein the N current transformers are respectively installed on short circuiting devices of a load screen, transmit detected values of grid insulation of branch circuits to the AD converter for conversion through the corresponding low-pass filters and then communicate with the information monitoring host or the PC through a CAN bus and an Ethernet interface. The current transformers detect fault current, perform related calculation processing and logic judgment on the fault current, and transmit a judgment result to an operator on duty through the information monitoring host or the PC, so that a grid insulation fault point is known in time. The digital AC grid insulation monitoring instrument is high in insulation monitoring accuracy and improves the grid power supply reliability.