Abstract: Distance resolution and system SNR are directly affected by FM characteristic of the Linear Frequency Modulation Continuous Wave（LFMCW） radar level gauge.In this paper，an evaluation function for frequency modulation linearity is constructed by using the spectrum characteristic of beat frequency signals.Later，Immune Mind Evolutionary Algorithm（IMEA） is introduced to optimize the end coordinates of the subsections to get nonlinear curve of frequency modulation voltage so as to compensate for the nonlinearity of VCO.Experiments show that the proposed method has good performance with no additional requirement for hardware and time cost，and the linearity and Signal Noise Ratio（SNR） of the system has been improved prominently.