Under short filament condition, the plasma with locally high density can be generated by means of the interaction between laser and medium from jet target, the acute change of dual-color-field phase difference is realized in a short distance, and the elliptically polarized terahertz (THz) radiation is easily obtained. By adjusting the back pressure of gas, THz radiation with ellipticity of 0.438 is generated. The simulated result based on the transient photocurrent model is consistent with that from the experiment, which reveals the mechanism of combined action of photocurrent and plasma dispersion.