Ptychography　can　reconstruct　the　complex　amplitude　distribution　of　the　transmission　object　from　the　overlapped　diffraction　patterns.　The　extended　ptychographical　iterative　engine　(ePIE)　algorithm　is　considered　as　one　of　the　most　popular　phase　retrieval　algorithms　of　present　time　which　has　strong　robustness　and　high　image　quality.　However,　no　constraint　is　added　in　the　iteration　process,　which　results　in　slow　convergence　and　makes　the　real-time　imaging　difficult.　To　overcome　this　problem,　an　efficient　phase　retrieval　algorithm　is　proposed　based　on　the　constraint　of　positive　absorption　in　the　object　plane　where　the　absorption　coefficient　and　phase　distribution　of　the　object　are　constrained　by　the　positive　absorption　of　the　object.　The　simulation　demonstrated　that　the　proposed　algorithm　has　greatly　improve　the　convergence　rate　and　imaging　quality　of　traditional　ePIE　algorithm　and　is　expected　to　be　applied　in　real-time　imaging.