A mixed-ligands complex Mn（mnt）（bipyO2）with charge-transfer transition from one ligand to the other was synthesized by the reaction of bipyO2（2，2′-bipyridine-1，1′-dioxide），Na2 mnt（maleonitriledithiolate）and MnCl2.The determinations by means of elemental analysis，infrared spectra，UV-Vis spectra，molar conductivity and TG-DTA showed that the complex was electrically neutral.It was thermostable brown solid below 310℃.The electronic spectra of the complexe in DMSO solvate exhibited intensive absorption bands at 250—290 nm（π-π* in bipyO2 and mnt），360—390 nm（π-π* in mnt）and intensive absorption bands at 410—430 nm was assigned as a metal-mnt π to bipyO2π* charge-transfer transition.The study of sensitivity properties showed that the complex was a good sensitive material for ammonia.The sensor made from the complex has good sensitivity and selectivity to ammonia.at working voltage 15 V and NH3 concentration in the range of （6.0×10-5）—（8.5×10-4）mol·L-1.The value rate of recovery of the sensor was 100.3% in average.The response time of the sensor was about 40 s，and the return time was about 45 s.The sensor could be used for detecting trace ammonia.