A temperature sensor based on erbium-doped fiber cascaded peanut taper (EDFCPT) structure in fiber ring laser (FRL) cavity is proposed and studied. The cascaded peanut structure was fabricated by erbium-doped fiber. The cladding mode and core mode interference and the strong thermo-optical effect of erbium-doped fiber were used to realize the high sensitivity measurement of temperature. The mode interference and thermal sensitivity effects of EDFCPT in a broadband super-continuous light source and a fiber ring laser cavity were studied and compared experimentally. Besides, Erbium-doped fiber cascaded peanut (EDFCP) structure was designed to compare performance with EDFCPT. The experimental results show that EDFCPT has a higher signal to noise ratio (SNR) (50dB) and a narrower 3-dB bandwidth (0.12nm) than the broadband light source. Among them, the temperature sensitivity of EDFCPT is 571pm/°C which is two times higher than the EDFCP (249nm/°C) structure at range from 5°C-55°C. The proposed temperature sensor has the advantages of high sensitivity, good repeatability, simple fabrication, compact structure, etc. It has a good application prospect in the field of aerospace and life health monitoring.