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YZ Phe is a very short-period contact binary (Sp.= $K2\,V$) with an orbitalperiod of 0.2347 days near the short period limit (0.22 d). We present thecomplete light curves in $VRI$ bands, which photometric data were obtained withthe 0.61-m telescope of PROMPT-8 at CTIO in Chile during June to October 2016and August 2017. The photometric solutions were determined by using the W-Dmethod and the results reveal that YZ Phe is a W-subtype shallow contact binary($f\sim$ 10%, $q$ = 2.635 or $1/q$ = 0.379 for W subtype) with rotationalmotion of a large hot spot on the more massive component, showing a strongO'Connell effect with variation of maxima in photometric time series at periodof 4.20 yr and stellar cycle at period of 1.28 yr. By compiling all availableeclipse times, the result shows a long-term period decrease at a rate of$\mathrm{d}P/\mathrm{d}t = -2.64(\pm 0.02)\times 10^{-8}$ d $yr^{-1}$,superimposed on a cyclic variation ($A_3$ = 0.0081 days and $P_3$ = 40.76years). This variation cannot be explained by Applegate mechanism. Thus, thecyclic change may be interpreted as light-travel time effect via the presenceof a cool third body. Based on photometric solutions, the third light wasdetected with 2% contribution of total light in $V$ and $I$ bands. Thosesupport the existence of a third body. For the long-term period decrease, itcan be explained by mass transfer from the more massive component ($M_2 \sim0.74 M_{\odot}$) to the less massive one ($M_1 \sim 0.28 M_{\odot}$) or plusAML via magnetic braking. With $1/q$ $<$ 0.4 and long-term period decrease, allsuggest that YZ Phe is on the AML-controlled state and its fill-out factor willincrease, as well as the system will evolve into a deeper normal contactbinary.

YZ Phe: an active contact binary with variation of the O'Connell effect and orbital period change