The invention provides a semi-open type reclamation sludge mudflat and a mudflat building method thereof. The mudflat structurally comprises a built seawall, a newly-built siltation-promotion submerged dyke, a hydraulic reclamation block and a hydraulic reclamation pipeline, wherein the built seawall is connected with the newly-built siltation-promotion submerged dyke, the hydraulic reclamation block is arranged between the built seawall and the newly-built siltation-promotion submerged dyke, and the hydraulic reclamation pipeline penetrates through one side of the built seawall to be connected with the hydraulic reclamation block. The newly-built siltation-promotion submerged dyke comprises a siltation-promotion submerged dyke body, a levee crown, a levee slope and a water-permeable structure. Compared with a traditional technology, under the conditions that a closure zone has not been closed, one side or two sides are opened, and water inside the closure zone mutually exchanges withwater outside the closure zone, on the dependence of an existing shoreline or dyke, the submerged dyke for reducing water power is properly built so as to achieve semi-open type hydraulic reclamationor naked blowing, and therefore, the aim of increasing mudflat resources is achieved. Meanwhile, through the natural gradient of dredger fill, the good ecological mudflat with high, middle and low beaches is formed. The technology is simple, the mudflat building speed is high, and the cost is economical; dredged spoil matter can be utilized, and the mudflat meets ecological green building requirements.