The aim of this study was to investigate the essence of deoxy-liquefaction technology by the comprehensive analysis of several aquatic plants' (AP) deoxy-liquefaction from the element and energy points of view. Accompanied by the oxygen removal in the form of CO2 and H2O, a great amount of hydrogen and carbon was shifted into water and char, leading to the low yield of liquid oil (LO) (11.23-16.70%). However, the residue carbon and hydrogen were adjusted to form mainly gasoline/diesel oil fractions and phenols. The deoxy-liquefaction was a similar miniature of the underground evolution from biomass to petroleum and coals during hundred thousands of years. The energy lost in the system was similar to 5.6% and similar to 19.9% based on the elemental balance of Nelumbo nucifera leaves (AP1) and Arundo donax (AP6). The closed system with the pressure of 8-10 MPa and the temperature of 350-400 degrees C made the decomposition pathway different from the traditional thermal conversion.