Linear ion trap mass analyzer could improve analytical performance compared to the traditional three-dimensional ion trap. In this study, a systematic investigation of the effect of octopole field on the performance of linear quadrupole ion trap was studied by both theoretical simulations and experiments. An asymmetric semi-circular linear ion trap (AsC-LIT) analyzer was designed and tested based on the theoretical results. The AsC-LIT consists of one pair of larger semi-circular rods with radius R_y and one pair of smaller semi-circular rods with radius R_x (R_y > R_x) in the cross section for introducing the quantitative octopole field. The analytical performances of these simplified AsC-LITs with different amounts of octopole fields were carefully investigated using both theoretical simulations and experimental validation. Experimental results showed that it could have a mass resolution of 1645 at m/z 175 (arginine); furthermore, the low mass cutoff and CID efficiency can be significantly improved when some positive octopole is added. The simulative results indicated that 2.56% octopole component was the best choice for achieving higher analytical performances. The theoretical and experimental studies of this work could certainly enrich the knowledge of a higher order electric field effect on the linear ion trap, furthermore, the simple geometric design and performance characteristics of this new AsC-LIT make it particularly attractive in the development of miniaturized mass spectrometers.