Rubidium isotope analysis provides critical insights into the origin and history of samples， with significant applications in geology， biomedicine， and forensic science. Wavelength modulation-direct absorption spectroscopy （WM-DAS） offers advantages of high signal-to-noise ratio and calibration-free operation， enabling rapid， high-sensitivity measurements of gaseous metal atom isotopic abundances. In this study， RbCl was used as the sample， and electrothermal reduction was employed to generate gaseous Rb atoms. The WM-DAS technique was integrated with threshold denoising to enhance signal quality. Absorption profiles of Rb atomic D₂ transitions were recorded using DAS， WM-DAS， and denoised WM-DAS methods， followed by Gaussian fitting. Signal-to-noise ratios were improved by factors of 1.40 and 1.84， respectively， with an additional enhancement of 1.48 times upon accounting for the hyperfine structure of the D₂ line. The temperature of gaseous Rb atoms was determined to be 439 K， corresponding to a saturated vapor pressure of 1.826 Pa. The isotope ratio of ⁸⁵Rb to ⁸⁷Rb was measured as 2.613±0.007， achieving a precision of 0.779%， superior to that obtained by DAS alone. The detection limit for ^⁸⁵Rb was established at 0.23%， satisfying the requirements for rubidium ore analysis. The combination of WM-DAS with electrothermal reduction demonstrates high precision and robust interference resistance， rendering this method highly suitable for field measurements and promising for isotope abundance analysis.