The invention discloses an intelligent substation process layer optical fiber transmission delay measuring method comprising the following steps: an IED sends a message; an ICU or MU responds to the message; and the IED compares the sent message with a received message to acquire optical fiber transmission delay. The message adopts an ultra-short frame, and has an identifier and a CRC (Cyclic Redundancy Check). The message only records one counter field. The data content corresponding to each frame of message can be calculated directly based on the data content of a previous frame of message. The counter field is related only to a local clock of equipment, and is unrelated to an absolute clock. An IED sends an ultra-short frame message; an ICU or MU responds to the ultra-short frame message; and the IED compares the sent message with a received message to measure optical fiber transmission delay. By using the method of the invention, error in signal time precision acquired by an IED caused by optical fiber length difference can be measured effectively in order to provide a data basis for compensating the signal time precision subsequently and improving the performance of the IED.