Abstract: The Serpens filament, as one of the nearest infrared dark clouds, is regarded as a pristine filament at a very early evolutionary stage of star formation. In order to study its molecular content and dynamical state, we mapped this filament in seven species including C18O, HCO+, HNC, HCN, N2H+, CS, and CH3OH. Among them, HCO+, HNC, HCN, and CS show self-absorption, while C18O is most sensitive to the filamentary structure. A kinematic analysis demonstrates that this filament forms a velocity-coherent (trans-)sonic structure, a large part of which is one of the most quiescent regions in the Serpens cloud. Widespread C18O depletion is found throughout the Serpens filament. Based on the Herschel dust-derived H2 column density map, the line mass of the filament is 41±5Msun/pc, and its full width at half maximum width is 0.17±0.01 pc, while its length is ~1.6 pc. The inner radial column density profile of this filament can be well fitted with a Plummer profile with an exponent of 3.0±0.2, a scale radius of 0.037±0.002 pc, and a central density of (2.2±0.1)e4 cm^{-3}. The Serpens filament appears to be slightly supercritical. The widespread blue-skewed HNC and CS line profiles and HCN hyperfine line anomalies across this filament indicate radial collapse in parts of the Serpens filament. C18O velocity gradients also indicate accretion flows along the filament. The velocity and density structures suggest that such accretion flows are likely due to the longitudinal collapse. Both the infall rate (~72 Msun/Myr, inferred from HNC and CS blue-skewed profiles) and the accretion rate (~10 Msun/Myr, inferred from C18O velocity gradients) along the Serpens filament are lower than all previously reported values in other filaments, indicating an earlier evolutionary stage.