The　traditional　simplified　model　for　ground　pressure　calculation　is　widely　used　in　the　anti-seismic　design　of　shallow　tunnels.　However,　it　presents　two　obvious　deficiencies,　which　are　related　to　the　block　equilibrium　and　the　additional　seismic　load　respectively.　A　modified　calculation　model　is　proposed　in　this　article　by　introducing　the　concept　of　slip　angle,　so　that　the　deflection　of　potential　slip　planes　and　the　distribution　of　ground　pressure　can　be　considered　and　further　discussed.　By　combining　the　horizontal　slice　method　and　graphical　method,　a　series　of　explicit　formulas　for　calculating　ground　pressure　are　presented　so　that　the　vertical　ground　pressure,　the　rupture　angle,　and　the　lateral　thrust　can　be　calculated　more　accurately.　The　proposed　modified　model　is　compared　with　the　existing　model　to　validate　the　correctness　of　calculation　results.　The　results　show　that　the　explicit　formula　of　modified　model　could　be　reduced　to　Liu＇s　formula　when　the　potential　slip　planes　are　assumed　to　be　vertical.　Additionally,　the　sensitivity　analyses　on　the　vertical　ground　pressure,　block　volume,　and　distribution　pattern　of　the　lateral　thrust　are　carefully　investigated.　The　lateral　thrusts　acting　on　the　left　and　right　wedges　distribute　logarithmically　and　exponentially　respectively,　which　implies　that　the　distribution　pattern　is　mainly　affected　by　the　direction　of　earthquake　inertia　force,　while　the　cohesion　and　the　friction　angle　merely　influence　the　magnitude　of　lateral　thrust　to　some　extent.