The invention discloses an active brazing device which comprises a water-cooling furnace body, wherein a furnace wall of the water-cooling furnace body adopts a hollow structure provided with a water inlet and a water outlet; a suspension device with a lifting function is arranged on a top wall of the water-cooling furnace body; a quartz crucible is suspended in an inner cavity of the water-cooling furnace body through the suspension device; an electric heating device is arranged in the water-cooling furnace body; a vacuumization device and a pneumatic pressurization device arranged outside the water-cooling furnace body are communicated with the inner cavity of the water-cooling furnace body through a vacuum pipe and a pressurization pipe; a valve is arranged on the pressurization pipe. The invention further discloses a brazing method for preparing a diamond-copper composite material with good interface bonding property and high thermal conductivity by using the device. The active brazing device has the advantages of simple structure, convenience for operation and low production cost; the preparation method is easy to control and the obtained diamond-copper composite material is good in interface bonding property and has the thermal conductivity reaching 480-500 W.m<-1>.K<-1>.