Doppler broadening and coincidence Doppler broadening of annihilation radiation experiments have been performed in three kinds of polyethylene glycol (PEG) membrane formed with different average molecular weight using the tunable monoenergy slow positron probe as a function of implantion energy.The obtained positron annihilation parameters are interpreted from two aspects:surface effect and differences in micro-structure or chemical environment of positron annihilation.The experimental results show that the regulation of densification of PEG molecular packing and distribution uniformity from the near surface layer to the bulk region in the film forming process can be well realized by changing its molecular weight.Combining a variable monoenergetic slow positron beam and these two positron annihilation spectroscopy methods is a powerful tool to study positron annihilation characteristics and for polymeric thin-film fine structure analysis.