Poly(allylamine phosphate) (PAP) was synthesized by solution polymerization using allylamine phosphate (AP) as monomer, 2,2’-azo-bis-2-amidinopropane dihydrochloride (AAP•2HCl) as initiator, respectively. PAP reacted with concentrated hydrochloric acid and it converted into poly(allylamine hydrochloride) (PAH). The effects of varying the concentrations of initiator and monomer on the polymerization conversion were investigated in detail. The chemical structure and thermal property of the polymer were studied by Fourier transform infrared spectrometer (FTIR), nuclear magnetic resonance spectrometer (NMR), thermogravimetric analyzer (TGA), differential scanning calorimeter (DSC), X-ray diffractmeter (XRD), respectively. PAH was prepared because of the disappearance of the stretching vibration absorption peaks and deformation vibration absorption peak of C-H bonds in -C=CH2 at 3020cm-1, 3085cm-1 and 1310cm-1 in the FTIR spectra. The three peak areas and their chemical shifts were consistent with the three kinds of hydrogen atoms in polymer formula in 1H NMR spectrum, which proved that PAH was synthesized. PAH had three decomposing stages and it decomposed completely at 700oC. The glass transition temperature (Tg) of PAH increased with decreasing concentration of initiator. The conversion of monomer increased with the increasing concentrations of initiator and monomer.