A new mechanism between the temperature sensitivity and the length ratio of PMMA coating to no-core fiber (NCF) is reported to realize an optical fiber temperature sensor with ultra-high sensitivity and compact size by PMMA-coated no-core fiber. By both theory and experiment, it is found that the sensitivity has a linear response to the length ratio of PMMA coating to NCF rather than the conventional viewpoint that it depends on the length of PMMA. Based on this conclusion and the high thermo-optic coefficient of PMMA, the temperature sensitivity is significantly enhanced as high as -9.582 nm/℃ through a simple, compact, and inexpensive sensor with 5 mm NCF and 3 mm PMMA coating. Our work opens a new avenue of a significant increase in the detection sensitivity of miniaturized fiber temperature sensors.