An in-depth understanding of debris flow erosion is crucial for estimating debris flow magnitude and evaluating its risk. Although the mechanisms of debris flow erosion have been widely explored by previous studies, channel downcutting and lateral erosion of debris flows over irregular erodible beds are still poorly understood. Here, we investigate the debris flow erosion characteristics in non-uniform erodible beds with variable cross-sections based on a series of small-scale flume experiments. We quantify the effects of debris flow density, bed slope, bed fine particle content, and channel narrowness on downcutting and lateral erosion. Downcutting Erosion Depths (DED) first increased linearly with the debris-flow density and bed slope; however, it exhibited a decrease afterward and an increasing trend with the narrower channel. The maximum DED was observed when the channel narrowness was 0.7. Similarly, a total erosion efficiency was introduced to measure the intensity of debris flow erosion. The efficiency increased linearly with the flume slope and decreased with the fine particle content of the bed and debris flow density. On the other hand, the efficiency grew exponentially with the channel narrowness. A dimensionless parameter, defined as the ratio of downward erosion to lateral erosion, was utilized to assess the relative importance of the downward and lateral erosion of the debris flow. Experimental results show that the ratio is positively correlated with debris flow density and flume slope. Notably, the effect of bed fine particle content was relatively weak, and downward erosion overwhelmed lateral erosion for significant channel narrowing. Furthermore, the ratio increased logarithmically with the channel narrowness, whereas the lateral erosion exceeded the downward erosion for smaller narrowness and vice versa. Our experimental results demonstrate that the amount and mechanism of debris flow erosion may differ significantly in irregular erodible beds.