According to the principles of simultaneous equilibrium and electronic charge neutrality, the thermodynamics in the system of SnCl4-NH4CI-HCl-H2O at normal condition has been calculated. Relation between all sorts of complex of Sn4+ and pH has been plotted. Based on thermodynamics analysis and calculation, some experiments was done to validate the relation between the total concentration of tin ion and pH in this system. Results suggest that the total concentration of ammonium and pH are the most important factors which determine whether (NH4)2SnCl6 or Sn(OH)4 exists in this system. Results further suggest when [HCI]≥6mol·l-1,Sn4+ in this system will be also precipitated in the form of (NH4)2SnCl6.These results lay the solid theory foundation to prepare pure (NH4)2SnCl6,a promising substitution for SnCl4·5H2O to prepare antimony doped tin oxide(ATO) and indium tin oxide(ITO),from the tinny material.