The light guiding mechanism of negative curvature hollow core fiber is analyzed based on anti-resonant reflected optical waveguide model and mode coupling model. Influence of structural parameters of circular tube negative curvature hollow core fiber on confinement loss of fundamental mode is simulated based on finite element method. Reduction reason of confinement loss of fundamental mode with nested tubes is considered, which is found that air layer between nested tubes has contribution to anti-resonant reflection, and nested tube has different potentials for confinement loss optimization under negative curvature fiber structure with different number of tubes. A negative curvature hollow core fiber with core diameter less than 20 μm is proposed with confinement loss less than 1 dB·km -1 at 1550 nm, and single mode transmission can be maintained.