A comprehensive numerical study on the effects of pulsating heating on the dynamics of the system of a Natural Circulation Loop (NCL) is obtained here. The NCL used as a passive heat transfer system in nuclear power plants and in other sectors of energy. It plays a pivotal role as an effective system for removal of heat. There are several challenges which are related with the operation of NCL. Flow instability in NCL is one of the toughest difficulties for the researchers. So, the topic draws more attention over past few decades. The system dynamic behaviour of several external pulsating power is provided and the system dynamics are observed. The current study focuses on a systematic characterization on the effect of fluctuating external power which is time-dependent for a single-phase Natural Circulating Loop. Numerical simulations are done on MATLAB based Simulink model. An oscillating heater power of sinusoidal in nature is given over a range of forcing amplitudes and frequency ratios to the system and the effect of these parameters are observed and simulated. The system dynamics is observed with the help of Time series plots, Phase plots, Fast Fourier Transformation plots (FFT), Largest Lyapunov Exponent and Correlation dimensions. Also, we used Recurrence plots to revalidate the system dynamics.