Disclosed herein is a method of enhancing speech. The method includes calculating a far-end speech spectrum by performing fast Fourier transformation of a signal received by a far-end user, calculating a background noise spectrum collected by a microphone provided to a mobile device of a near-end user; calculating a gain from the far-end speech spectrum and the background noise spectrum using a speech intelligibility index-based module, and deriving an enhanced far-end speech spectrum by applying the gain to the far-end speech spectrum, wherein, in calculating a gain using a speech intelligibility index-based module, a power budget used for transmitting and receiving a speech signal is set to vary with the background noise spectrum.