The invention aims to discloses a safety shell inside main steam pipeline leakage monitoring system which comprises a plurality of micro optical fiber probes mounted at leakage monitoring points in asafety shell, a spectrum analyzer based on direct absorption method TDLAS technology and a digital display device for real-time display. The micro optical fiber probes and the spectrum analyzer are inmutual communication connection through armored optical fiber, and the digital display device and the spectrum analyzer are in mutual communication connection. Compared with the prior art, the systemhas the advantages that multiple monitoring points of gaseous-phase leaking steam is directly monitored in environment of the safety shell, higher main steam pipeline leakage monitoring sensitivity and quicker response time are realized, response speed after leakage is found is remarkably increased, bigger safety allowance is provided, and the objectives of the invention are achieved.