The mechanism of activity loss oi RuO2-TiO2 anode (DSA) in saturated NaCl solution was studied in terms of electrochemistry and surface physics. The information obtained in different periods of electrolysis showed that the loss of the activity of the DSA took place gradually. The contents and the structure of the RuO2-TiO2 containing layer changed continuously and the dissolution of the Ru component was main reason for the degradation of the DSA. The titanium hydroxide precipitated on the electrode surface also accelerates the loss of activity oi the DSA. The cracking corrosion in the coating layer was another reason for the activity loss. No obvious evidence was found to prove the happening of passivation of the DSA.