2011年，美國政府在經歷次級房貸和高軍事支出的雙重壓力下，爆發高度財政赤字的問題，造成歐巴馬政府面臨調高債務比例與債務上限的壓力。然而，在眾多的輿論聲中，美國民主黨與共和黨在八月底達成下列協議，減少政府支出、提高債務比例以及增加債務上限等；但是，是否這些方式將改善美國經濟？本篇文章在動態隨機一般均衡(DSGE)架構下，建立一個封閉經濟體系，並將政府支出加入私人廠商部門，透過公共投資，幫助私人廠商增加產出；並且在政府僅採行公債和徵稅融通下，找出一個最適的債務持有比例，使國內福利為最高。而本文發現政府進入生產部門時，將影響最適債務持有比例。即是，隨著政府支出生產彈性越大，最適債務持有比例也會上升，而在基準參數下，我們將會得到最適債務持有比例為百分之十的結論。 / In 2011, under the pressure of subprime mortgage and high military expenditure, the U.S. government accumulated high fiscal deficit, and the Obama government faced the pressure of raising debt ratio and raising debt ceiling. However, among the huge debates, the Republican Party and Democratic Party reached the deal in August which included cut-down government expenditure, raise debt ratio, raise debt ceiling, and so on. But, will these ways improve the U.S. economy? This paper follows the dynamic stochastic general equilibrium (DSGE) framework to construct a closed economy, which the government helps private firm to production through public investment. Besides, given that government only undertakes debt financing and tax financing, we try to find an optimal debt ratio which makes the highest domestic welfare. In our finding, if the government enters private production sector, the optimal debt ratio will be influenced. That is, the optimal debt ratio will increase with the production elasticity of government expenditure. Under the benchmark parameter, the optimal debt ratio is 10 percent.