Background <p>The current research on the performance of GaN pixel detector only focuses on the device itself, but the influence of the pixel size of GaN-based pixel detector on the energy spectrum resolution is noteworthy, because it is related to the energy spectrum acquisition efficiency.</p> Methods <p>In the first part, their C–V and I–V characteristics are compared. Through the forward I–V test results, we can see that the detector we made this time has excellent junction characteristics. The reverse I–V test shows that the dark current of the detector is at pA level, which proves that the detector has excellent performance. We used <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(^{241}\mathrm{{Am}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mmultiscripts> <mrow /> <mrow /> <mn>241</mn> </mmultiscripts> <mi mathvariant="normal">Am</mi> </mrow> </math></EquationSource> </InlineEquation> radiation source to illuminate detectors of different pixel sizes, and collect its spectrum to see whether different pixel areas affect the resolution of the detector. Through this study, we use thicker epitaxial layers as much as possible to improve the collection efficiency of particle detection.</p> Results <p>The results show that the size of the detector does not affect the resolution of collecting energy spectrum, and the counting rate of large-area pixels is very high, which tells us to use thicker epitaxial layer as much as possible to improve the collection efficiency of particle detection.</p>