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The equivalence ratio is a key parameter in understanding pyrolysis, combustion and gasification of fuels for improving combustion efficiency and lowering net pollution emissions. In this study, we investigate the feasibility of measuring the local equivalence ratio in a gaseous flow field using femtosecond filament excitation. We demonstrate that quasi-collimated-beam-induced filamentation in a premixed methane/air flow can produce a long and uniform free-of-atomic-line emission channel, and that the intensity ratios of the molecular emissions in this channel, such as methylidyne radical (CH) (331 nm)/N2 (337 nm), CH (331 nm)/N2 (357 nm), and CH (331 nm)/CN (388 nm), are all linear correlations to the equivalence ratios of the methane/air flow. Our results provide a possibility for the in situ measurement of long-range and multi-dimensional equivalence ratio of flow fields with femtosecond filament excitation. [ABSTRACT FROM AUTHOR]