Diode-pumped alkali vapor laser (DPAL) has great potential in high power scaling due to its high quantum efficiencies, convenient thermal management by flowing the gaseous medium, all-electric operation, light weight and compact configuration, etc. The measurement of alkali concentration inside the gain medium is necessary and important to analyze the laser performance. By scanning the wavelength of the single-frequency distributed Bragg reflector (DBR) laser, the absorption spectrum of rubidium vapor is measured. By using the spectral matching method which is realized by integrated adjustment of the DBR′s driven current and operation temperature, the mode-hop free tuning range of DBR laser is extended from 23 GHz to 100 GHz. And the full absorption spectrum of rubidium D1 line is well scanned with buffer gas at an atmospheric pressure. By compared with the theoretical result, the concentration of alkali atom is obtained. This method can be applied for the measurement of alkali concentration under conditions of high power pumping with a flowing medium.