According to ratio changes of reference and filtering light paths, changes of outdoor temperature are estimated by a demodulation system based on linear filters. In experiments, the influence from changes of outdoor temperature on its transmittance is eliminated by constant temperature control on linear filters. Electro-optical con. version is performed to the collected optical signals and AD collection module is used to collect. Ratios of two light paths, images for monitoring environment and video information are transmitted in radio by on-line monitoring systems through general packet radio service (GPRS) network finally. The collected information is transmitted to host PC for displaying and processing data and image information. And remote, real time and on-line monitoring on temperature in special environment is realized by the demodulation system.