A high precision and accuracy wavelength tunable system for a modulated grating Y-branch (MGY) tunable laser is built. The high-precision wavelength scanning and calibration are realized by coarse scanning and fine scanning. Based on this, the temperature drift characteristics of the output wavelength and the internal Fabry-Perot (F-P) etalon are tested. It is found that there exists a good linear relationship between the output wavelength of this MGY laser and temperature, whose slope is constant. The intercept varies linearly with temperature. A high precision and accuracy wavelength calibration method based on external gas absorption reference is researched and its experimental test is conducted. The research results show that within the temperature range of -25-+75 ℃ and the wavelength scanning range of 40 nm, the established system possesses a tuning curve linearity better than 0.9999, a tuning wavelength precision better than 0.18 pm, and an accuracy better than 0.12 pm.