In order to study the immunity of calibration-free wavelength modulation spectroscopy to laser intensity change and external interference, multi-path absorption spectra based on calibration-free wavelength modulation technology were adopted. Taking acetylene as the measurement target, theoretical analysis and experimental verification were carried out. The results show that, the amplitude of wavelength modulated second harmonic signal varies significantly under different laser power. But the signal obtained by the calibration-free method changes little. It is less affected by external disturbance such as airflow, partial shading and system vibration. The spectral signals with volume fraction in the range of 5×10-6 ～9×10-5 obtained by calibration-free method have good linearity. The correlation coefficient is 0.9997. Allan variance analysis shows that the minimum detection limit of the experimental system can reach 1.2×10-8. This study shows that calibration-free wavelength modulation spectroscopy can avoid the interference of light intensity jitter, air flow and system vibration. The system stability and detection sensitivity are improved.