Among many terahertz wave generation methods, nonlinear optical collinear difference frequency generation (CDFG) is always regarded as a promising way to achieve the high power, broadband, continual tunable terahertz wave emission. Theoretical analysis shows that a big coherent length (on millimeter scale) in isotropic GaP crystal could be realized by the laser wavelength near 1064 nm in the CDFG process, which meets the condition of high power and broadband terahertz generation. In the experiment, a high power and broadband terahertz wave is achieved from GaP crystal, with its tunable terahertz range 95.9-773.4 μm (0.39-3.13 THz) and the highest terahertz peak power 7 W at 2 THz. The experimental result is generally consistent well with its theoretical calculation.