A power efficient optical frequency comb (OFC) is highly desired for applications with limited power supply, e.g., microwave photonics applications in moving vehicles. An ultraflat OFC generation is demonstrated with 0.19-dB flatness, 15 comb lines, and 18.3-dBm total RF power consumption. The scheme is based on spectral convolution. A five-line comb with 0.06-dB flatness and 4.0-GHz spacing is first generated in an intensity modulator. Then the signal is injected to a phase-modulator driven by frequency of 20.0 GHz to obtain the ultraflat 15-line comb with 0.19-dB flatness. The comb spacing is adjustable by controlling the input frequency of microwave source within the bandwidth of phase-modulation. Our work demonstrates a power efficient approach to obtain ultraflat comb and may benefit microwave photonics applications requiring low-power consumption.