Diffusion models have received a lot of attention in the field of generation due to their ability to produce high-quality samples. However, several recent studies indicate that treating all distributions equally in diffusion model training is sub-optimal. In this paper, we conduct an in-depth theoretical analysis of the forward process of diffusion models. Our findings reveal that the distribution variations are non-uniform throughout the diffusion process and the most drastic variations in distribution occur in the initial stages. Consequently, simple uniform timestep sampling strategy fail to align with these properties, potentially leading to sub-optimal training of diffusion models. To address this, we propose the Beta-Tuned Timestep Diffusion Model (B-TTDM), which devises a timestep sampling strategy based on the beta distribution. By choosing the correct parameters, B-TTDM aligns the timestep sampling distribution with the properties of the forward diffusion process. Extensive experiments on different benchmark datasets validate the effectiveness of B-TTDM.