Very minimum chirp keying (VMCK) is a highly efficient ultra-narrowband modulation technique, which has the advantage of narrow band occupancy, intensive sideband suppression, and higher data transmission efficiency. However, if we directly derive the analytical expression of its power spectrum from VMCK signal expression, the result is complex and imprecise because the result contains a non-integral function. Linear spectrum does not carry useful information. In order to obtain narrower bandwidth, stronger sideband suppression and better demodulation performance, we remove the harmonic spectral lines in power spectrum by separate sinusoidal basis fitting to optimize VMCK.