A problem in the lattice orderdecision-making theory,i. e., the structure of lattice order is
incomplete, was investigated. By defining the regulation ofdetermining key elements and theminimal
decision se,t a method was given to directly work out join-and meet-irreducible elements of the
smallest complete lattice from a partial order se.t The inverse order embeddingmappingmethod was
used to determine the greatest lowerbound or the leastupperbound of two elementswithoutthe greatest
lowerbound or the least upper bound to obtain the lattice order of incomplete lattice. Finally, an
examplewas given to illustrate the feasibility of thismethod.