This　paper　proposes　a　rectangular　approximation　method　for　curved-shape　power　density　to　rapidly　calculate　the　temperature　profile　of　the　entire　chip　by　using　the　2D　thermal　model　with　the　effective　thermal　characteristic　length.　The　proposed　method　employs　rectangular　blocks　to　approximate　the　non-rectangular　power　density　areas,　thereby　enabling　the　utilization　of　stepwise　integration　for　the　determination　of　the　cosine　series　coefficients.　In　contrast　to　the　conventional　grid　mesh　approach,　this　rectangular　approximation　significantly　reduces　the　number　of　required　mesh　elements,　thus　considerably　enhancing　computational　efficiency.　Numerical　results　reveal　that　the　proposed　method　achieves　a　speed　improvement　ranging　from　94x　to　195x　over　the　traditional　grid　technique,　while　maintaining　comparable　accuracy.　Furthermore,　the　maximum　absolute　error　observed　in　temperature　predictions　is　limited　to　a　mere　0.37K.