As the core equipment of medium and large-capacity new energy power generation system, the modulation algorithm of the type I neutral point clamped (NPC) three-level (3L) converter can directly affect the energy conversion efficiency, harmonic characteristics and operation control characteristics of the converter. In view of that lots of trigonometric function computation is included in the implementation process of the traditional space vector pulse width modulation (SVPWM) strategy of type I NPC 3L converter, this paper executes the sector-area location identifications of the reference voltage vector and the active time calculations of the voltage vector adopted in the modulation algorithm based on the three-phase stationary coordinate system. By induction and reorganization, the three-phase unified duty ratio function available for all sectors is obtained according to the carrier-based idea of single modulated wave with dual-carrier. With the duty ratio function, the digital controller of the converter can obtain the control signals of the power switches by simple relational operations and numerical calculations, which avoids a lot of trigonometric function calculations and saves hardware resources. In addition, the SVPWM and sinusoidal pulse width modulation (SPWM) + 3rd harmonic injection modulation algorithms are compared, and the intrinsic differences between the two modulation algorithms from the perspective of the modulated waves are revealed. Finally, the effectiveness of the proposed algorithm is verified by software simulation.