Due to the electromagnetic interference, the onboard electronic measuring equipment will cause malfunction or malfunction. The traditional chaotic spread spectrum can reduce the electromagnetic interference of the onboard LLC resonant converter from the source, but with the increase of the spread spectrum width, the output voltage ripple will increase and the efficiency will decrease. Based on this, firstly, the chaotic sequence is generated by a lattice multivortex chua′s system, and the chaotic spread spectrum is realized in the way of leftward spread spectrum. Secondly, proposing a segmental traversal optimization algorithm with the rate of change of electromagnetic interference rejection and the rate of change of spreading width as the criterion, combine the prediction model of conducted interference with highfrequency parasitic parameters, and perform the spreading width optimization for different frequency bands specified in the national standard. Finally, the simulation and experimental results verify that the proposed strategy can achieve the maximum 7.07 dB EMI reduction, the maximum 61.5% output voltage ripple reduction, and the circuit efficiency improvement.