Multistage flash (MSF) desalination technique is one of the simplest of thermal desalination methods which requires thermal energy in order to desalinate seawater. This thermal energy can be provided by solar energy harnessed by a direct absorption solar collector (DASC) in which a nanofluid while flowing through the collector absorbs the incident irradiation directly and gets heated to higher temperatures. These collectors are having a relatively higher thermal efficiency (10% higher) as compared to conventional surface-absorption-based solar collectors. In this study, a direct absorption solar collector (DASC) has been used as a heat source for multistage flash (MSF) desalination system having once-through (OT) configuration, and these two systems are coupled using a counterflow type heat exchanger. This direct absorption collector is replaced by surface-absorption-based collector in order to prevent the degradation of thermal performance of surface-absorption-based collector due to high salinity of seawater as in the present case seawater flows through heat exchanger and is getting heated by the nanofluid flowing through direct absorption collector. The aim of the present study is to evaluate the thermal performance of the combined system which is represented by a quantity known as gained output ratio (GOR). The thermal performance or efficiency of the solar collector depends upon various parameters such as thickness of nanofluid layer inside DASC (H), length of the collector (L), particle volume fraction of nanoparticles (f_v), and incident solar energy (q) which will affect the performance of the MSF system also. Hence, the performance of the combined system will be evaluated as a function of the collector parameters mentioned above. The gained output ratio is also evaluated as a function of brine rejection temperature (T_b) and feed seawater temperature (T_f) which are parameters associated with the MSF desalination system. The fresh water production rate (ṁ_d) has also been evaluated as a function of the abovementioned parameters related to the collector and MSF system. A numerical model has been prepared to solve the temperature profile of the DASC system which is solved using finite difference implicit method (FDM) with the help of MATLAB. The numerical model for MSF desalination system is also prepared and solved in MATLAB.