We demonstrate power-based refractive index (RI) sensing using an ultrasonically crushed polymer optical fiber (POF). This structure can be fabricated easily and cost-efficiently for a short time (of ∼1 s) with no need to employ external heat sources or chemicals. What is required is an ultrasonic transducer, the horn of which is pressed against part of the POF. The RI dependence of the transmitted power is investigated, and almost linear trends are obtained in the RI ranges of from ∼1.32 to ∼1.36 (coefficient: -62 dB/RIU (RI unit)) and from ∼1.40 to ∼1.44 (coefficient: -257 dB/RIU)). When the RI is fixed at 1.33, the temperature dependence of the transmitted power is also investigated (coefficient: -0.094 dB/°C), which shows the temperature-dependent error in RI sensing can be compensated.