  By performing angle-resolved photoemission spectroscopy and first-principles
calculations, we address the topological phase of CaAgP and investigate the
topological phase transition in CaAg(P1-xAsx). We reveal that in CaAgP, the
bulk band gap and surface states with a large bandwidth are topologically
trivial, in agreement with hybrid density functional theory calculations. The
calculations also indicate that application of "negative" hydrostatic pressure
can transform trivial semiconducting CaAgP into an ideal topological nodal-line
semimetal phase. The topological transition can be realized by partial
isovalent P/As substitution at x = 0.38.
