In conventional drum-type maize threshing, the tightly arranged kernels on the maize ear lead to a high inter-kernel support force during the initial stage of threshing, resulting in strong impacts from the threshing elements and severe kernel damage. To address this issue, a pre-threshing principle for maize ears was proposed. This approach involved pretreating the ear before threshing by inducing internal splitting of the cob from the inside out into fragments, thereby disrupting the original compact kernel arrangement, loosening the kernels, and significantly reducing the inter-kernel support force. Based on this principle, a novel internal cob-splitting pre-threshing device for maize ears was developed. The device consisted primarily of an actuating cylinder, a prismatic splitting wedge, and an ear-holding fixture. Through analysis of the ear-splitting process, key structural parameters were identified, including the number of prism edges, operating air pressure, and fixture spacing. Single-factor experiments and Box-Behnken response surface tests were conducted to investigate the influence of these factors. The results showed that the significance of each factor affecting the proportion of small fragments and kernel breakage rate followed the descending order as follows: number of prism edges, fixture spacing and air pressure. The optimal combination of parameters was determined to be 8 prism edges, 2cm fixture spacing, and 0.55MPa air pressure. Under this configuration, five validation tests were conducted, yielding an average small fragment proportion of 74.38% and a kernel breakage rate of 1.07%, closely aligning with the predicted values. The results confirmed that the device can effectively split the maize ear into fragments with a high proportion of small cob pieces, while maintaining low kernel damage and significantly reducing the inter-kernel support force. Comparative threshing tests were carried out between a pre-threshing unit with a reciprocating flexible threshing unit and a 5TY-45-150 thresher, and a 5TY-45-150 thresher fed directly with the whole cob. The results showed that the threshing method of “pre-threshing + 5TY-45-150 thresher” and the threshing method of “pre-threshing + reciprocating flexible threshing device” were comparable to the threshing method of feeding the whole cob directly into the 5TY-45-150 thresher. The seed breakage rate of the threshing method was reduced by 2.73 percentage points and 2.97 percentage points, respectively. The reductions were as high as 55.94% and 60.86%, respectively. It proved that the pre-threshing device can significantly improve the operation quality of the whole threshing process. The research result can provide an approach and technical support for improving the quality of mechanized maize threshing.