Abstract: Ultrasound thermal strain imaging (TSI) employs tracking of echo shifts in ultrasound images to measure in-tissue temperature changes during thermotherapies. The accuracy of thermometry is susceptible to a number of factors, including the thermoacoustic lens effect, physiological motions, and the tissue type. As it is difficult to calibrate the temperature distribution within tissues, it is essential to develop simulation models to evaluate the efficacy of different TSI algorithms. This paper puts forward a methodology for simulating ultrasound images during thermotherapies. This is based on the finite-difference time-domain (FDTD) solution of the bio-heat transfer equation, in conjunction with the theory of thermally-induced echo shifts. In constructing the FDTD model, the stability and computational efficiency of explicit, implicit and alternating direction implicit (Douglas-ADI, DADI) methods are compared, with DADI being identified as the preferred scheme. Subsequently, the theoretical echo offset during tissue heating was calculated on the basis of the temperature-dependent speed of sound and the thermal expansion of the tissue. Subsequently, the echo shift distribution is assigned to the in-tissue scatterers during ultrasound imaging. The efficacy of a TSI algorithm is evaluated by performing thermometry on the generated image sequence, revealing the limitations of the algorithm, the sensitivity of different tissues to TSI thermometry, and the negligibility of thermal expansion. Furthermore, the performance of the TSI algorithm is examined in the context of multiple heating sources and consideration of periodic physiological movements.