Abstract: It is of significant importance for DNA research that the effects of salt NaCl and polymer PEG on the denaturation behavior of salmon DNA were studied by UV measurements and relevant model to compare. The results show that the addition of salt has influences on both the electrostatic repulsions between DNA molecules and the hydrogen bonding of the base pairs, thus affecting the DNA melting temperature. The DNA melting temperature in the presence of PEG is lower than that in the pure DNA aqueous solution, owing to the competition between the crowding effect caused by PEG and the binding of PEG with DNA. The effect of PEG on DNA melting temperature becomes stronger with increasing PEG molecular mass. The density functional theory is applied to study DNA denaturation theoretically. The reaction thermodynamic theory is made to examine the features of bulk denaturation and the denature temperature and denature curve are calculated by the minimum free energy principle. The results can precisely predict the denaturation behavior in NaCl and PEG solutions. The melting temperature is calculated by the established molecular thermodynamic model and the calculated melting temperature matches fairly well with the experimental results.