To meet the forestry units needs for maximizing the economic benefit of timber cultivated by them under the circumstance of market economy, the paper tries to set up a new mathematical model of the optimal logging age of timber. Based on the theory of supply demand balance of microeconomics and by the methods of economic analysis and mathematical inference, after analyzing the advantages and disadvantages of both (1) the mathematical model of the optimal logging age of timber in the hope of getting the highest average annual timber output [A (t) =T ′ (t) =M (t) ] and (2) the mathematical model of the optimal logging age of timber in an attempt to get the highest present value of the timbers future value on the hypothesis that the timbers price is constant [i=T ′ (t) /T (t) =M (t) /T (t) ], the paper further advances the mathematical model of the optimal logging age of timber in order to get the highest present value of the timbers future value on the hypothesis that the timbers price is variable [i=T ′ (t) /T (t) +P ′ (t) /P (t) = M (t) /T (t) + P ′ (t) /P (t) ]. This new mathematical model will supply the forestry units with a more scientific and feasible quantitative method to decide the optimal logging age to attain the optimal economic benefit under the circumstance of market economy.