In this work, we study the spectral property of a class of self-affine measures μ
M,D
on
R
2
generated by an expanding real matrix
M
=
diag
ρ
1
−
1
,
ρ
2
−
1
and a non-collinear integer digit set
D
=
{
(
0
,
0
)
t
,
(
α
1
,
α
2
)
t
,
(
β
1
,
β
2
)
t
}
with
α
i
−
2
β
i
∉
3
Z
, i = 1, 2. We give the sufficient and necessary conditions so that μ
M,D
becomes a spectral measure, i.e., there exists a countable subset
Λ
⊂
R
2
such that {e
2πi⟨λ,x⟩: λ ∈ Λ} forms an orthonormal basis for L
2(μ
M,D
). This extends the results of Dai, Fu and Yan (2021 Appl. Comput. Harmon. Anal.
52 63–81) and Deng and Lau (2015 J. Funct. Anal.
269 1310–1326).