A colorimetry catalyzed by gold nanoparticles (AuNPs) for mercury ions detection assisted by the nonthermal microplasma is developed. The AuNPs are synthesized in situ with the microplasma generated on the pneumatic nebulization. The size of the AuNPs can be controlled in a certain range. The “all-in-one” system is designed for sample introduction, AuNPs synthesis, and mercury ions detection. The 3, $3^{\prime}$ ,5, $5^{\prime}$ -tetramethylbenzidine (TMB) reagent is catalyzed by the synergistic effect of AuNPs and mercury ions, and the solution in detection cell turns blue. The mechanism for Hg2+ detection, the catalytic role of the AuNPs, and the condition for redox reaction are investigated in this work. The system shows the linear range of 0–500 nmol L-1 for mercury ions detection with the limit of detection (LOD) of 10 nmol L-1. The integrate “all-in-one” system is portable and convenient for the detection in emergency compared with the commercial instruments. Finally, some practical samples are determined to verify the accuracy, reliability, and practicability of the present system.