For the combining torsional buckling of small size Carbon Nanotubes (CNTs) in the multi-field coupling condition, a mechanical model was established in a couple field based on nonlocal theory and the combining torsional buckling behavior of the model was studied. Firstly, the constitutive relation was introduced under thermo-electro-mechanical loadings by using an elastic shell model of continuum mechanics. Then, the nonlocal elasticity theory was induced to discuss the scale effect of the small size carbon nanotubes, meanwhile, in consideration of the van der Waals forces and the effect of surrounded elastic medium, the generalized governing equation of buckling for CNTs was established in the multi-field coupling based on nonlocal theory. With applied torque and torsion-related axial load, as well as changed temperatures and voltage loads, the influences of different factors on the combined torsional buckling behaviors was calculated numerically. Finally,the conclusion demonstrates the response of the combined torsional buckling of small size CNTs in the multi-field coupling condition,and reveals the relation between combined torsional buckling of small size CNTs and each field. Moreover, it shows that the ratio of critical buckling load between nonlocal and local theories is always smaller than 1, which indicates that the torsional buckling of small size CNTs is overestimated in classical theory.