Although　maximum　displacement　demands　of　bridges　can　be　obtained　through　non-linear　time　history　analysis,　many　approximate　methods　have　been　proposed　in　recent　codes　to　reduce　the　required　computational　time　distinctive　of　non-linear　approaches.　Among　these　methods,　equivalent　linearization　method　is　best　known.　This　paper　evaluates　the　accuracy　of　equivalent　linearization　method　adopted　in　AASHTO　through　comparison　of　non-linear　and　linear　time　history　analysis.　The　isolation　system　is　characterized　by　bilinear　hysteretic　behavior　and　the　bridge　is　simplified　to　be　a　single-degree-of-freedom　(SDOF)　system.　A　comprehensive　parametric　study　is　performed　using　a　program　developed　by　MATLAB　in　combination　with　OpenSees.　Results　show　that　the　predicted　maximum　displacement　is　significantly　underestimated　by　AASHTO　method　when　the　strain　hardening　ratio　and　ductility　ratio　are　relatively　small.　To　improve　the　estimation　accuracy　of　equivalent　linearization　technique,　a　new　equation　of　equivalent　damping　ratio　is　proposed　by　modifying　AASHTO　method.