As terahertz technology is widely used in spectroscopy, imaging, medicine and other fields, the demand for high-quality terahertz light sources is becoming increasingly apparent. Compared with traditional terahertz generation technology, the technology of femtosecond laser-induced gas plasma to generate terahertz waves has attracted widespread attention due to its advantage of being able to generate strong terahertz waves without damage threshold. Among them, the two-color laser filamentation method has the advantage of high conversion efficiency in the infrared to terahertz band, which greatly increases the energy of terahertz waves. However, it took a long time for the physical mechanism behind it to be understood based on the semi-classical theory of transient photocurrent model. According to previous studies, terahertz wave mainly originates from the radiation generated by the asymmetric photocurrent caused by the two-color field excitation plasma process, however, plasma will strongly modulate the terahertz wave. Therefore, in the actual terahertz generation process, both plasma modulation and photocurrent radiation play an important role. At present, many research groups have carried out work related to plasma modulation of terahertz, however, there are relatively few studies on the comparison of the contribution of plasma modulation and photocurrent radiation to terahertz generation.This paper first gives a theoretical model in three-dimensional conditions, including Maxwell's equations describing the propagation effect, the photocurrent equation describing the generation of photocurrent and plasma modulation, the density evolution equation describing the change of free electron density, and the tunneling ionization equation describing the ionization rate, and details the main conditions and related parameters used in this paper.In the simulation part, we first study the dielectric ionization and terahertz generation process at the center point based on the finite difference time domain algorithm, clarify the synchronization of terahertz generation and plasma step ionization, and confirm that terahertz wave will inevitably drive the newly generated free electrons synchronously with the two-color field, thereby modulating the terahertz wave. Subsequently, by studying the forward and backward terahertz generation under near-field conditions, it is found that ignoring the modulation of plasma on photocurrent radiation will cause it to have a maximum at zero frequency, while introducing plasma modulation on photocurrent radiation will eliminate the radiation maximum at zero frequency. This is because the modulation of plasma on photocurrent radiation greatly suppresses the frequency components below the plasma frequency. In addition, the introduction of plasma modulation of photocurrent radiation causes a certain degree of blue shift in the radiation spectrum. This is because the photocurrent radiation accelerates the vibration of free electrons in the plasma, which in turn causes the overall radiation spectrum to move toward the high frequency direction. Further, by studying the forward and backward terahertz generation under far-field conditions, it is found that when the modulation of plasma on photocurrent radiation is not considered, the forward radiation spectrum has a smooth single-peak structure, while the backward radiation spectrum presents a series of equally spaced modulation structures, and this phenomenon is accurately explained by the principle of coherent superposition. When considering the modulation of the radiation wave by plasma, it is found that the components of the forward far-field radiation and the backward far-field radiation below the plasma frequency are significantly suppressed due to the shielding effect of the plasma, while the components above the plasma frequency are enhanced to a certain extent, which is also caused by the photocurrent radiation accelerating the vibration of free electrons in the plasma.Based on the calculation and analysis of terahertz generation under different conditions, this paper confirms the low-frequency suppression and high-frequency blue shift and enhancement of terahertz wave by plasma modulation, points out the incompleteness of the explanation of coherent superposition of single-point radiation, and proposes a three-step model to more comprehensively explain the physical mechanism of terahertz generation, deepen the understanding of terahertz wave generated by laser-induced plasma, and has a good guiding role in experiments.