Abstract: On-line water dew point analyzer and portable water dew point analyzer are used at Lunnan Initial Compressor Station of West-to-East Gas Transmission Pipeline aiming at the high water dew point of the coming gas and the high fluctuation which has great impact on the operation of the pipeline and gas transmission equipment. The readings of natural gas's water dew point from the two kinds of analyzers are quite different. Comparison and analysis are made to investigate the differences and impact factors. The results show that the two kinds water dew point analyzers have different work scopes and measuring errors.and the former is suitable for continuously measuring the variation of gas's water dew point.