1. A radio frequency temperature sensor, comprising: a ring oscillator, adapted to generate an oscillating signal frequency varying corresponding to a measured temperature; a memory, adapted to store an initial value; a frequency counter, coupled to the memory and the ring oscillator and adapted to measure a temperature error in response to the measured temperature according to the initial value and the oscillating signal in a pre-set time; a radio frequency transmission interface, serving as a transmission interface with a card reader; and a micro-controller, coupled to the frequency counter, the memory and the radio frequency transmission interface, adapted to transmit the initial value to the frequency counter, control starting and ending of the pre-set time, read the temperature error and communicate with the card reader through the radio frequency transmission interface.