We consider a series of independent observations from a
P
-norm distribution with the position parameter
μ
and the scale parameter
σ
. We test the simple hypothesis
H
0
:
σ
=
σ
1
versus
H
1
:
σ
=
σ
2
. Firstly, we give the stop rule and decision rule of sequential probabilistic ratio test (SPRT). Secondly, we prove the existence of
h
σ
which needs to satisfy the specific situation in SPRT method, and the approximate formula of the mean sample function is derived. Finally, a simulation example is given. The simulation shows that the ratio of sample size required by SPRT and the classic Neyman–Pearson
N
−
P
test is about
50.92
%
at most,
38.30
%
at least.