  {\small We investigate nonclassical properties of the field states generated
by subtracting any number photon from the squeezed thermal state (STS). It is
found that the normalization factor of photon-subtracted STS (PSSTS) is a
Legendre polynomial of squeezing parameter }${\small r}${\small \ and average
photon number }$\bar{n}$ {\small of thermal state. Expressions of several
quasi-probability distributions of PSSTS are derived analytically. Furthermore,
the nonclassicality is discussed in terms of the negativity of Wigner function
(WF). It is shown that the WF of single PSSTS always has negative values if
}$\bar{n}<\sinh^{2}r${\small \ at the phase space center. The decoherence
effect on PSSTS is then included by analytically deriving the time evolution of
WF. The results show that the WF of single PSSTS has negative value if
}$2\kappa t<\ln\{1-(2\bar{n}+1)(\bar{n}-\sinh^{2}% r)${\small
}$[(2\mathfrak{N}+1)(\bar{n}\cosh2r+\sinh^{2}r)]\}${\small, which is dependent
not only on average number }$\mathfrak{N}${\small \ of environment, but also on
}$\bar{n}$ {\small and }$r${\small . }
