Abstract: Natural capital assessment is an important way to study ecosystem services and socioeconomic development. Due to the particularity， complexity， openness， fluidity and multilevel coupling of the ocean， the methods for the assessment of marine natural capital develop slowly. The emergy analysis method converts different types of energy and substances， including the flowing and the stored， in the ecosystem into the same standard emergy through the transformity in order to measure and compare different levels of energy value. Thus it can completely reflect the real natural capital value which characterized by no other methods. To accurately quantify the value of marine life capital， this paper introduces the energy flow diagram of marine food web and the law of energy transfer in ecosystem， then proposes an empirical formula of organisms transformity， namely T_n=T₁·E^1-n_L（where T₁ is the solar transformity of the primary producer，n is the trophic level， and E_L is the Lindeman efficiency）. In order to prove the feasibility of the empirical formula， with cases studied we obtained some trophic levels and corresponding transformities of organisms in different feeding habits of a marine food web. Due to inherent error within the empirical formula and for a better accuracy, practically the value of the key parameters should be determined by literature and data surveys. The empirical formula obtained in this study simplifies the transformity calculation method and promotes the further application of emergy analysis in marine natural capital assessment．