Let the base radius be r and the bus length be l.

Know the meaning of the question,

The circumference of the bottom surface is 2π r.

Because it is unfolded into a semicircle,

So 2π r = 1/2 (2π l)

Then 2 π r = π l

l/r=2

That is, the ratio of the generatrix length to the radius length of the cone bottom surface is 2.