Based on non-collinear phase-matching (PM), a new method for widely tunable wavelength was proposed and demonstrated in a four-mirror ring cavity of non-critical phase-matching (NCPM) KTiOAsO4 (KTA) optical parametric oscillator (OPO). Wavelength tuning range of 141 nm from 1535.56 nm to 1676.73 nm was achieved by moving one mirror of ring cavity back and forth. The tuning theory and tuning method of non-collinear PM were analyzed in detail. The output energy and pulse width of signal were measured and compared in collinear and non-collinear PM condition. This method is also applicable to OPOs of other nonlinear crystals based on four-mirror ring cavity.