Terahertz (THz) radiation generation by two-color femtosecond laser filamentation is a promising path for high-intensity THz source development. The intrinsic characteristics of the filament, especially its length, play a crucial role in determining the THz radiation strength. However, a detailed analysis of the underlying physical mechanism and the quantitative correlation between the laser filament length and the THz radiation intensity under a high-peak-power driving laser is still lacking. In this paper, the effect of filament length on the THz radiation is investigated by modulating the basic characteristics of the two-color laser field and changing the focal length. Experimental results show that the long filament length is advantageous for improving THz radiation intensity. The theoretical simulation indicates that enhancement of THz radiation arises from coherent accumulation of THz wave produced at each cross-section along the filament. These insights suggest that extending the filament length is an effective scheme to enhance the intensity of THz radiation generated by the two-color femtosecond laser filament.