What is the result unit?

The 1 MeV neutron is used to bomb Li-6 target with 100 um, Usryield card is used to tally 6Li(n,t) double different cross section, the comment of Usryield card is:

and the result is:

the value of Y-axis is so little, what is its unit?

The Y-axis values make no sense, because you asked for d2sigma/dx1dx2. The concerned sigma is the beam particle inelastic cross section, corresponding to the inelastic scattering length printed in the material table of the output file (where by the way you picked up by default hydrogen, since you did not specify the material), which however does not apply to low energy neutrons, thereby getting zero cross section (< 1e-30) and infinite scattering length (> 1e30).
Just ask for the recommended d2N/dx1dx2 (for which no material has to be specified) and you will get double differential triton yield per incident neutron, which you can independently rescale to a cross section value.

@ Francesco Cerutticeruttif, thanks!

@ceruttif, yes your are right, here is the result:

through the cross section at 90° to experimental data, it is right.

but in the result, the cross section below 30° is so little, this is because the cross section used in fluka is lack of some data?

No, it’s because you put the triton energy cut at 4 MeV. Raise it to 4.2 MeV and you will fill also the low polar angle part.

At 90°, the value of Y-axis is 3.63e-3, so the different cross section is 3.63e-3/(1/66.021e23*0.01)=1.45e-26cm2=14.5mb, the data of cross section from iaea is 15.5mb,
the data of cross section from iaea at 19° is 35.6 mb,

so the value of Y-axis should be more 7e-3, but on the result figure the value is so little.

See above.

@ceruttif, yes, you are right, thanks!