The open system pyrolysis characteristics of 7 low rank coals and the evolution kinetics of H2 and its relation to the first coalification jump were determined using a thermogravimetric analyzer coupled to a quadrupole mass spectrometer (TG/MS). Results showed that both the coal pyrolysis characteristics and the evolution kinetics of H2 are strongly related to the first coalification jump, which is demonstrated by three aspects: the weight loss rate indicates an abrupt change at Cdaf=80%(R0max =0.60%) (Cdaf is carbon content (mass fraction) of sample in dry-ash-free basis, R0max is the maximum reflectance of vitrinite in coal); the characteristic temperatures and kinetic parameters of H2 exhibit a minimumvalue at Cdaf=80% (R0max =0.60%); the total yield of hydrogen had a maximum value at Cdaf=80% (R0max=0.60%) and this is consistent with the initial point of the first coalification jump. These aspects indicate that the derived parameters can reveal the first coalification jump. Combined with the structural characterization of the coals by FTIR, a mechanismthat explains the results was determined using the following three factors: (1) the evolution characteristics of aliphatic oxygen-containing functional groups; (2) the retrogressive reaction of soluble organic matter and (3) the "polarization" of organic matter.