A novel method with high accuracy and easy implementation was proposed based on the sinewave-scanned direct absorption spectroscopy (DAS) in this paper. A fitting routine in the time domain was developed to simultaneously deduce the baseline and more importantly, absorbance through the explicit baseline expression offered by the sinewave scan. This method effectively solves the difficulties of baseline determination and provides more accurate wavelength calibration compared with conventional DAS. The accuracy and performance with narrow scans and high frequency were experimentally verified using CO transition at 4300.699 cm-1, from which the inferred line strength agrees well with HITRAN 2016. Meanwhile, a more accurate N2 collisional broadening was provided and the speed-dependent collisional broadening coefficient of this transition was reported for the first time.