The development of cuffless blood pressure monitoring devices has been attracting attention in the field of medical devices. Measuring the diameter of blood vessels based on the A-mode waveform obtained by an ultrasound sensor attached to the body surface is suitable for daily blood pressure monitoring because of the simplicity of the device. However, movement by the user can cause device displacement, which reduces the received intensity of the wave reflected from the blood vessel wall, resulting in the incorrect measurement of the blood vessel diameter. To solve this problem, we previously proposed a blood vessel diameter sensor with an acoustic lens that transmits and receives ultrasound waves within a certain angular range along the long axis of the blood vessel, and verified the improved robustness of blood vessel diameter measurement compared with that with a planar sensor. The design of blood vessel diameter sensors requires the consideration of reflections and artifacts other than the blood vessel wall, and the use of simulation technology helps in efficient design. In this paper, we describe a simulation model for a blood vessel diameter sensor to support the design process. The simulation model was evaluated using an experimental blood vessel model (phantom) with characteristics similar to those for a human body, and the experimental and simulation results were found to be in agreement in terms of the angular range over which blood vessel diameters can be accurately measured and the effects of reflections and artifacts other than blood vessels. The simulation model is expected to accelerate our understanding of the effects of noise factors and the investigation of the causes of problems and improvements.