A fiberoptic intrusion detecting system bases on intermodal interference has been installed for field test. The line-sensing system demonstrates high sensitivity and capability to provide position information. It is shown to be able to detect an intruder walking at 16 meters from the buried fiber. Except during heavy raining , or when the field is being flooded, detection rate of crossing the sensing line, by walking, running or crawling, is 100%. Improvements on sensor design and method of installation to enhance reliability are discussed.