In　this　paper,　a　method　is　presented　to　obtain　both　in-plane　and　out-plane　dynamic　effective　parameters　of　plate-type　elastic　metamaterials　with　decentralized　resonant　mass.　The　method　is　derived　by　plate-type　elastic　metamaterials　with　concentrated　resonant　mass,　and　the　effective　material　properties　are　obtained　by　averaging　local　physical　field　in　each　phase　when　prescribed　displacements　are　applied　on　the　boundaries　of　the　unit　cells.　The　accuracy　of　the　method　is　verified,　since　the　frequency　ranges　of　negative　mass　density　are　the　same　with　those　of　band　gaps.　In　addition,　the　starting　and　ending　of　negative　mass　density　are　analyzed　by　eigenstates　on　the　bandgap　edges.　Multi-negative　parameters　are　produced　by　abundant　resonances　exhibited　in　the　unit　cells　when　synergetic　motions　are　generated　by　the　decentralized　resonant　mass.　The　proposed　method　is　extended　to　determine　the　effective　properties　of　the　plate-type　elastic　metamaterials　with　decentralized　resonant　mass.　Compared　with　the　calculated　effective　properties　of　the　model　with　concentrated　resonant　mass,　more　negative　mass　density　regions　are　formed　both　in-plane　and　out-plane　for　the　model　with　decentralized　resonant　mass.　Moreover,　negative　moduli　are　also　generated　by　monopolar　and　quadrupolar　resonances,　and　the　negative　hybrid　dispersion　bands　are　produced　by　the　combination　of　negative　mass　density　and　negative　moduli.　The　calculated　results　show　that　the　proposed　method　is　convenient　to　determine　both　in-plane　and　out-plane　dynamic　effective　parameters　of　plate-type　elastic　metamaterials　with　complex　internal　structures,　and　easier　than　the　method　based　on　the　Mie　scattering　solution.　Therefore,　the　proposed　method　will　be　helpful　in　designing　plate-type　elastic　metamaterials　with　resonant　micro-structures.