Sulfomethylated phenolic resin (SMP) is a drilling mud additive having good fluid loss and dispersion characters and may be resistant to the action of high temperature and high salt frequently encountered in well drilling, especially in deep wells. SMP is one kind of polyelectrolyte. There are dissociable groups (-CH2SO3-Na+) along the chain of SMP. The conformations and properties of SMP in solution are affected greatly due to electrostatic attraction and repulsion. From the experimental results of viscosities and fluorescence spectra of polyelectrolyte SMP in pure water and aqueous solutions containing added salt, the effects of added salt on molecular conformations of SMP in aqueous solution and properties of reducing fluid loss of water base drilling mud are discussed.