The riserless mud recovery system can realize the recovery and recycling of mud under open drilling conditions， which has the advantages of safety and environmental protection， simplified wellbore structure and drilling risk reduction. This technology has been developed abroad for many years and successfully promoted in the industrial application， however， China is still in the stage of tracking research. This paper introduces the design and development process of first 400m riserless mud recovery system in China， and verifies the functions and indicators of the system through sea trial tests. The results show that the prototype of the developed equipment is equipped on the “Marine Geology-10” ship， the equipment loading is reasonably adapted， the maximum sea test depth is 384.7m， the functions of the equipment meet the design requirements， the identification and automatic control function of mud level are successfully realized， and the feasibility of riserless mud recovery and circulation drilling technology is verified in the whole process.