A frequency offset calibrating method for use in a communication device connected to a communication system is provided. The method includes the following steps: determining a discontinuous reception cycle; awakening the communication device to a working mode from a sleep mode every discontinuous reception cycle and keep the communication device in the working mode for a first time period to receive a paging indication channel message from a communication network periodically; and awakening the communication device at a second time period other than the first time period during a first discontinuous reception cycle, thereby estimating an accumulated timing offset of a clock signal of the communication device and calibrating a frequency offset of the clock signal. In the invention, the accumulated timing offset of the clock signal can be calibrated efficiently to increase the reception performance of the page indication channel message with simple implementation and low hardware cost.