We theoretically present a highly efficient optical method to obtain slow and fast light in a coupled system consisting of a nanomechanical resonator and quantum dots in terms of mechanically induced coherent population oscillation (MICPO). Turning on or turning off the specific detuning of pump field from exciton resonance, this coupling system can provide us a direct optical way to obtain the slow or fast group velocity without absorption. Our coupling scheme proposed here works as a fast and slow-light knob and may have potential applications in various domains such as optical communication and biology sensor.