Cr4　+　∶　YAG　crystal　materials　have　been　proved　to　be　an　ideal　passive　Q-switched　material　in　recent　years.　Using　MgO　and　CaO　as　sintering　aid　and　charge　compensation　agent,　0.　25%　Cr4　+　∶　YAG　(molar　concentration)　transparent　ceramics　with　high　optical　quality　were　successfully　prepared　by　low-temperature　vacuum　sintering　combined　with　hot　isostatic　pressing　(HIP)　sintering.　Effect　of　MgO,　CaO,　and　MgO-CaO　co-added　divalent　oxides　on　the　transmittance,　absorption　spectra,　and　fluorescent　properties　of　Cr4　+　∶　YAG　ceramics　were　investigated.　It　is　found　that,　compared　with　CaO　and　MgO-CaO　series,　MgO　exhibits　the　most　significant　effect　on　the　densification　of　Cr4　+　∶　YAG　ceramics　when　the　vacuum-sintering　temperature　is　1　550　℃　.　Especially　when　the　molar　concentration　of　MgO　is　0.　25%,　the　relative　density　reaches　98%　.　After　HIP　sintering　at　1　680　℃,　the　corresponding　in-line　transmittance　of　the　densified　Cr4　+　∶　YAG　ceramics　reaches　as　high　as　84.　4%　at　the　wavelength　of　1　400　nm.　Moreover,　by　annealing　at　1　400　℃　for　20　h,　the　absorption　coefficient　of　Cr4　+　at　1　030　nm　reaches　the　highest　value　of　3.　0　cm　-　1　,　though　some　Cr3　+　still　remain　in　the　Cr4　+　∶　YAG　ceramics.　©　2024　Chinese　Ceramic　Society.　All　rights　reserved.