Using the type Ⅰ matching mode of negative uniaxial crystal and with the microanalysis method, the angle-tunable curve that indicates the connection between wavelength increment of signal light and phase matching angle is studied. Under the condition of a little of phase-matching angle-variety, the proximate angle-tunable curve is gained. After constructing a function which is token of intensity of the birefringence property, principally, the relation between the birefringence property of crystal and wavelength-tunable sensitivity relative to increment of phase matching angle is discussed. The result shows that the angle-tunable curve is very sensitive when the birefringence property of crystal is stronger.