Geo-technical investigation of the Shiziyang main channel in the Guangdong-HongKong-Macao Greater Bay Area cannot be carried out with vertical point drilling. Therefore， an extended-reach novel horizontal borehole with high horizontal displacement/vertical depth ratio was designed for investigation： designed hole depth of 940m， 14 coring targets， target radius of 5.0m， horizontal section length of 380m， vertical depth of 60m， and horizontal displacement/vertical depth ratio of 14. It is one of the boreholes with the most working targets per single hole and the highest horizontal displacement/vertical ratio in China. A set of comprehensive exploration technologies， including multi-target directional drilling， undisturbed coring by the mud motor， geological logging and drill string-conveyed storage logging have been developed， and applied for the first time in water conservancy and hydropower investigation with good results. The completed drilling depth of the hole was 932.20m， and the target hit rate was 85.71% with the coring footage of 81.71m， the penetration rate 2.38m/h， and the core recovery rate 89.84%. With the novel drilling technologies， accurate advance geological prediction of the main channel on the ocean floor was achieved with great reduction of the number of exploration holes， less environmental damage， higher exploration efficiency； and the continuity and reliability of geological data was ensured. It is an advanced technology worth popularizing in tunnel exploration.