This paper introduced the design principle and method of the coaxial line low pass filter, and designed a coaxial low pass filter with 4 GHz cutoff frequency, which structure named structure No.2 is that the inner conductor’s diameter is unified and the outer conductor’s diameter is varied in stepped variation. The structure No.2 has the advantages of simpler design, smaller size and better performance over the structure No.1, which outer conductor’s diameter is unified and the inner conductor’s diameter is varied in stepped variation. The test results showed that the filter had good performance with the insertion loss of less than 0.4 dB, return loss of more than 15 dB in the whole pass band. At 6 GHz outside the passband, the superession is about 36 dB, which satisfied overall the design requirements.