Experimental research and numerical analysis were applied to study the ultimate load capacity (ULC) and reinforcement of circular-hollow-section N-joint. Four specimens were tested under static load. The ULC of each specimen was obtained and the detailed failure conditions were discussed. Based on the results, both welding a plate on the chord member and filling concrete in the chord member are effective to reinforce the N-joint, but it is suggested that these two methods should not be applied simultaneously. Moreover, considering the four important influence factors of ULC of the joints, the ratio of chord diameter to chord thickness, the ratio of diagonal member diameter to chord member diameter, the axial load on the chord member and the gap size between the two brace members, a new prediction formula of ULC of N-joint was put forward based on numerical analysis results. The new formula was proved to be more accurate than the current Chinese design code in a wide range.