  In this paper we study blow-up and lifespan estimate for solutions to the
Cauchy problem with small data for semilinear wave equations with scattering
damping and negative mass term. We show that the negative mass term will play a
dominant role when the decay of its coefficients is not so fast, thus the
solutions will blow up in a finite time. What is more, we establish a lifespan
estimate from above which is much shorter than the usual one.
