本文主要研究了估值效应如何调节外债对经济增长的影响。以往的研究大多忽视汇率问题，但由于绝大部分外债是以外币计价的债务，自然面临汇率风险，产生估值效应，因此有必要考虑汇率对外债使用效率的影响。本文将汇率变动因素纳入到外债对产出的边际效应影响的分析框架中：首先，通过理论模型，推导出汇率如何调节外债对产出的边际影响，以及汇率如何影响一国最优规模的变动，并指出汇率变动主要是通过影响该国的外债负担，从而影响外债对产出的边际效应与最优外债规模；其次，本文还使用了2002年至2019年118个国家的经济数据，通过面板数据模型对相关数据进行回归分析，以验证理论模型的结论。通过分析，本文的结论如下：第一，汇率变动率与外债的边际效应为负相关关系：外币贬值，即估值效应为正，会加强外债对产出的扩张效应和减弱外债对产出的抑制效应；反之，则减弱外债对产出的扩张效应和加强外债对产出的抑制效应；第二，随着估值效应由正转负，外债对产出的边际效应由扩张效应转为抑制效应。如果不存在估值效应，则外债对经济起着抑制效应，外币汇率升值会加强抑制效应，而外币汇率贬值会减弱抑制效应，并可能将抑制效应调整为扩张效应；第三，汇率变动与最优外债规模呈现负相关关系，即外币汇率的上升会引起最优外债规模的下降，导致经济体面临更大的外债风险，外债更可能对GDP产生抑制效应，对经济体产生负面影响。

This paper mainly studies how the valuation effect affects the impact of external debt on economic growth. Previous studies mostly ignored the exchange rate, but most external debt is denominated in foreign currency, so it suffers exchange rate risk and valuation effect. Therefore, it is necessary to consider the impact of exchange rate on the efficiency of external debt. This paper adds the factors of exchange rate change to the analysis framework of the marginal effect of external debt on output: Firstly, through the theoretical model, this paper shows how the exchange rate affects the marginal effect of external debt on output and how the exchange rate affects the change of a country's optimal scale, and points out that the exchange rate change mainly affects the marginal effect of external debt on output and the optimal scale of external debt by affecting the country's external debt burden; Secondly, this paper also uses the economic data of 118 countries from 2002 to 2019, and makes regression analysis on the relevant data through the panel data model to verify the conclusion of the theoretical model. Through the analysis, the conclusions of this paper are as follows: First, the rate of change of exchange rate is negatively correlated with the marginal effect of external debt: Foreign currency depreciation, that is, the valuation effect is positive, which will strengthen the expansion effect of external debt on output and weaken the inhibition effect of external debt on output; On the contrary, it weakens the expansion effect of external debt on output and strengthens the inhibition effect of external debt on output; Second, as the valuation effect changes from positive to negative, the marginal effect of external debt on output changes from expansion effect to inhibition effect. If there is no valuation effect, the external debt has a restraining effect on the economy, the appreciation of foreign currency exchange rate will strengthen the restraining effect, while the depreciation of foreign currency exchange rate will weaken the restraining effect, and the restraining effect may be adjusted to expansion effect; Third, there is a negative correlation between the change of exchange rate and the optimal scale of external debt, that is, the rise of foreign currency exchange rate will lead to the decline of the optimal scale of external debt, resulting in the economy facing greater external debt risk. External debt is more likely to have a restraining effect on GDP and have a negative impact on the economy.