Abstract: The unauthorized flight of unmanned aerial vehicles has become an increasingly frequent phenomenon in society. Featuring a low flight altitude， small volume， and small radar cross-section， such unmanned aerial vehicle targets present prominent low observability， which poses severe challenges to low-altitude supervision work. Nevertheless， the traditional multiple input multiple output （MIMO） radar structure is plagued by the drawbacks of a small aperture and low degree of freedom， making it difficult to meet the direction-finding requirements for such low-observability targets. Traditional MIMO radar structures suffer from limited aperture and low degrees of freedom （DOFs）， making it difficult to meet the multi-target observation requirements. To address this issue， this paper proposes a design strategy based on the monostatic coprime MIMO radar structure from the perspective of the postage-stamp problem （PSP） to improve the direction of arrival （DOA） estimation performance. The construction process of the difference co-array of sum co-array （DCSC） generally consists of two steps： calculating the second-order sum co-array （SC） of the receiving and transmitting arrays， and constructing the DCSC. The proposed design strategy first models the step of calculating the SC as a PSP and arranges physical sensors according to the solution of the PSP. Second， it introduces an expansion factor to expand the array aperture flexibly. Finally， it optimizes the expansion factor based on the principle of maximizing continuous DOFs. This study combines this design strategy with the padded coprime array to propose a padded coprime array-based MIMO radar. Simulation results verify the superiority of the proposed design strategy and MIMO radar structure in terms of DOA estimation.