Aligned arrays of semiconducting carbon nanotubes (s-CNTs) with high homogenous density and orientation are urgently needed for high-performance carbon-based electronics. Herein, a length-controlled approach using combined technologies was developed to regulate the s-CNT length and reduce the length distribution. The impact of different lengths and length distributions was studied during aligned self-assembly on a liquid–liquid confined interface was investigated. The results show that short s-CNTs with a narrow distribution have the best alignment uniformity over the large scale. The optimized and aligned s-CNT array can reach a density as high as 100 CNTs·μm⁻¹ on a 4-inch wafer. The field-effect transistor (FET) performance of these optimized s-CNT arrays was 64% higher than arrays without length-control. This study clarified that rational control of s-CNTs with desired length and length distribution on the aligned self-assembly process within the liquid–liquid confined interface. The results illustrate a solid foundation for the application of emerging carbon-based electronics.