This issue provides the basis for the opportunistic actions of managers to avoid disclosing negative news in line with their personal interests and hide them inside the company. When this mass of negative information enters the market, it causes the stock price to fall. Thus, the purpose of this research is to study the effect of auditor size on the relationship between financial constraints and risk of stock price fall. The statistical population of the research of companies admitted to the Tehran Stock Exchange, the systematic elimination sampling method and the studied sample, includes 152 admitted companies during the years 2017 to 2018. The descriptive-survey research method is a combination of causation-correlation in terms of the relationship between the variables, and it is practical in terms of purpose. To process and test hypotheses, regression method, panel data and fixed effects model were used. The findings of the hypothesis analysis showed that the size of the auditor has a negative and significant effect on the relationship between financial constraints and the risk of falling stock prices.