A widely tunable (5～12.5 μm) continuous wave mid-infrared laser based on difference frequency generation (DFG) in a AgGaS2 crystal with type II (e+o→e)phase matching is reported. The pump laser is a master oscillator power amplifier (MOPA) system which consists of a grating-tuned external-cavity diode laser in Littrow configuration and a tapered semiconductor amplifier, which is tunable from 760 to 790 nm with a maximum power of 800 mW(780 nm). The signal laser is a Tisapphire laser, which is tunable from 790 to 910 nm with a maximum power of 760 mW (806 nm). A maximum output power of 1.076 μW is obtained near 7.0 μm. Based on the difference frequency generation laser, the direct absorption spectrum of (000→010) band of water vapor near 7.0 μm in laboratory air is measured. Under the absorption optical path of 19.0 cm in open air, the concentration of water vapor in laboratory air is estimated from the recorded absorption spectrum.