Metallic bipolar plates (BPPs) are regarded as the most promising substitute of traditional carbon-based BPPs due to their good mechanical properties, electrical conductivity, high productivity, and low cost in mass production. However, conventional design guidelines on flow channels of carbon-based BPPs are not perfectly valid to metallic BPPs due to the formability of thin metallic sheets, which are at risk of material rupture especially when flow channels decrease. The objective of this work is to develop a forming limit model to establish the relationship between the channel height and channel geometric dimensions by the micro stamping process, and the maximum channel height can be predicted to guide channel design of metallic BPPs from the formability perspective. Firstly, an instability criterion for micro-scale forming was proposed to estimate when the rupture occurs during the forming process. Forming height as the function of channel features is established, and the limit of forming depth can be predicted. Series of micro stamping experiments with various dimensions are conducted to validate the accuracy of the model. Influences of main parameters on the channel forming limit and evaluation of channel design are discussed based on the model. The model in this study is an effective supplement to the channel design principle for metallic BPPs. Based on the model, design, fabrication, and testing of metallic BPPs will be done in the following research, Part II: Experiments.