A flat panel detector comprising: a first optical assembly (OPA1), having a first side and a second side opposite to the first side in a thickness direction of the flat panel detector, and comprising: a first scintillator layer (SCL1) configured for converting at least part of rays (X1) into a first visible light (L1, L2); and a first light guide component (OPS1) stacked with the first scintillator layer (SCL1) and configured for guiding the first visible light (L1, L2); a first image sensor assembly stacked with the first optical assembly (OPA1), configured for receiving the first visible light (L1, L2), and comprising: a first image sensor (M1) located at the first side of the first optical assembly (OPA1); and a second image sensor (M2) located at the second side of the first optical assembly (OPA1), characterized in that the flat panel detector further comprises: a second optical assembly (OPA2), located at a side of the first image sensor (M1) far away from the first optical assembly (OPA1), stacked with the first optical assembly (OPA1) and the first image sensor assembly and comprising a second scintillator layer (SCL2) configured for converting at least another part of the rays (X2) into a second visible light (L3, L4); a second image sensor assembly, stacked with the second optical assembly (OPA2), configured for receiving the second visible light (L3, L4), and comprising a third image sensor (M3) located on a side of the second optical assembly (OPA2) far away from the first optical assembly (OPA1). a first optical assembly (OPA1), having a first side and a second side opposite to the first side in a thickness direction of the flat panel detector, and comprising: a first scintillator layer (SCL1) configured for converting at least part of rays (X1) into a first visible light (L1, L2); and a first light guide component (OPS1) stacked with the first scintillator layer (SCL1) and configured for guiding the first visible light (L1, L2); a first scintillator layer (SCL1) configured for converting at least part of rays (X1) into a first visible light (L1, L2); and a first light guide component (OPS1) stacked with the first scintillator layer (SCL1) and configured for guiding the first visible light (L1, L2); a first image sensor assembly stacked with the first optical assembly (OPA1), configured for receiving the first visible light (L1, L2), and comprising: a first image sensor (M1) located at the first side of the first optical assembly (OPA1); and a second image sensor (M2) located at the second side of the first optical assembly (OPA1), a first image sensor (M1) located at the first side of the first optical assembly (OPA1); and a second image sensor (M2) located at the second side of the first optical assembly (OPA1), a second optical assembly (OPA2), located at a side of the first image sensor (M1) far away from the first optical assembly (OPA1), stacked with the first optical assembly (OPA1) and the first image sensor assembly and comprising a second scintillator layer (SCL2) configured for converting at least another part of the rays (X2) into a second visible light (L3, L4); a second image sensor assembly, stacked with the second optical assembly (OPA2), configured for receiving the second visible light (L3, L4), and comprising a third image sensor (M3) located on a side of the second optical assembly (OPA2) far away from the first optical assembly (OPA1). <EOS>