A suit of feasible online evaluation system is needed for check the actual function of fire alarm system of tunnel and underground space. The definition accordance of tunnel fire disaster is put forward based on experimental data of full-scale tunnel fire developing curves in China and abroad; the personalized phenomenon of operational tunnel alarm system is proved by measuring data by quasi-distributed fiber Bragg grating heat fire detector in tunnel; a rational threshold value range that meets the requirement of failed and false alarm rates is proposed for digital alarm system, and the range should be regarded as the evaluation index for the functional integrity of the system; the superior limit of the range can be determined by statistically analyzing the daily tunnel operational temperature data, and the lower limit can be determined by analyzing the fire temperature data under special conditions. The range can be updated in real-time.